I will probably get flamed for this but the following is true from my point of view as a male dating a female with children but could equally be applied to a single Dad.
I will give you my own personal take on it from experience of someone in their 30s who has been through it all.
If your a single guy with no kids then definetly do not date a single mum, you don't want to be playing step daddy role in your 20/30s when there are plenty of women out there who are child free and single and you can both go off care free and do what you want when you want with out having to arrange childcare.
You both will have different levels of freedoms/free time/ responsibilitys, their children will allways be number one and you will allways be second, you will have 7 child free nights each week, they may have one or two depending on childcare arrangements which you will need to organise your life around.
Believe me I have done it in the past, I'll give you some examples of what I mean and the hassle involved, if ever you try to arrange a mid week date, you have to wait for them to sort childcare out for their children, you get out have nice dinner then go have some drinks in a bar afterwards, things are going nicely, then they glance at their watch and are like it's half 8 I've got to go back and relieve the babysitter, put kids to bed get them ready for school in the morning, none of this staying out til late then going back to either one of your houses for some fun which is what dating is all about and what you would be able to do with someone who is child free.
Another common example is if they got their kids on weekends and you want to see them, you will have to spend the day down peppa pig world or some other child friendly place and then if you want to eat out you will have to go to places like the hungry horse and eat crap food whilst listening to their children running around screaming. It gets worse as they get older with stroppy attitude teenagers.
If You goto any special events and they bring their kids, your be the first ones leaving at 7pm when all of your friends will be having fun staying out with their partners having a few drinks enjoying the rest of the night.
I think it's fair to say that you will have alot more free time than someone who has got kids, do you really want to be waiting around not being able to see your partner because they have got to look after their kids, trust me you will get bored on weekends and all the week nights when you can't see them and you have the evening and weekends free to go out and do things and make memories together and they are not able to. You won't be able to just go out last minute for a meal out and stay out late, everything needs to be planned weeks in advance and even then you will only have a certain amount of time together.
If you want to goto festivals/night's out/birthdays/weddings /party's and they can't get childcare you will be expected to stay in with them or prepare to be branded selfish and be emotionally blackmailed into staying in on a Sat night with them and their children because they can't get childcare and it's not fair that you can go out and enjoy your self if they can't.
As for holidays, there won't be any last minute weekends/night's away or going away outside of summer holidays, you will be expected to go in the summer holidays to child family places like Butlins where it costs 4 times as much and crazy busy, no thankyou! Many blokes idea of hell!
Adult only places and activitys will be a thing of the past, no sitting around the pool for a quiet drink, or going exploring on adventure holidays.
Then there's usually the dramas with the exes and then it gets all complicated around Xmas and Birthdays ferrying kids from one house to the next to see their real Dads and Grandparents. Also how would you or your parents feel if you had to introduce your GF and her children to your parents for the first time, secretly they would not be impressed with your life decision and would wish for you to do much better.
Why invest all your time/money/effort into someone's else's kids. Trust me just find someone who is at the same life stage as yourself, not married, no kids, career e.t.c it will work out better long term, usually you will last the honeymoon period but after that the hassle/dramas/problems/lifestyle change will be the breaking point and you will have just wasted time in finding the right one for the long term, by all means if they are fit then treat them as a bit of fun but don't be getting involved you will only regret it at a later date.
If you have any ambitions of moving somewhere new or starting a life up somewhere else, forget it they won't be able to uproot their children and family ties for your wants.
Then there's the embarrassment of introducing her and her children to your parents for the first time, especially if your siblings have got their own biological children and you've picked up somebody else's kids, They won't tell you they are ashamed of you, but know that they are.
You will allways be second in the relationship, their children will allways come first, so you will treat her as your number one priority but unfortunately she will treat you second and you will be the one making the sacrifices in your lifestyle to accommodate.
Long term you will have to likely give up your hobbys to take there children to swimming lessons/scouts/brownies/football/school and wait around and pick them up if they are busy, if you can't/won't you will be emotionally blackmailed as you will be into deep by that time and will be obliged to help out with their children so not to cause any resentment towards yourself. It starts off as a once off favour and then a year later your doing it every week.
Also don't be duped on dating websites like tinder/bumble e.t.c when they say they are not looking for a step dad for their kids because they allready have a dad, that's all good and that but what happens in say 5 years time when you both want to live together, are their children going to live elsewhere or are you going to be living with some underage lodgers that you have absolutely no control over in your house because your not a step dad. There's a good reason why 90% of the women on there are single Mums, nobody wants anything to do with that whole situation.
Then there's the problem of if they need benefits in order to survive, if you ever move in together and you earn even a modest amount they will loose all their benefit entitlements because it is assessed on household income, so if they receive housing allowance, child benefits, child tax credits, working tax credits, reduced/no council tax, free childcare, free school meals e.t.c you will have to pay for it all which can be between £1000 - £1700 a month in situations I have been in as their take home from there partr time job is likely to be hardly anything.