I was 19 when I had my first. He was a novalty to friends, which quickly wore off when I would keep saying 'no I can't come out', 'no I can't help you move', 'no I won't be attending' etc.
18 years later, some are now having their first. I get 'it's so unfair you get to go away again?', 'no sorry I can't come out as I don't have a sitter', 'I'm really struggling with work and childcare' etc. And I will be honest, I do feel a little smug. I think yep now you know how I felt. I may be a little nasty, but I'm not at their beck and call, they weren't there when I needed them, so why should I make myself available 24/7 for them. Childish? Probably. But still carrying that hurt.
But I have to be honest, I haven't noticed having children affect my romantic relationships. I met now exdp when ds was 2, he had no children. He was an amazing 'step dad' as such, but as he said, he would never be a step dad to ds, because he has a dad, and we were 50/50 contact. Him and ex dp get along really well. We had a child together and split several months ago after 16 years, although the relationship was really over a few years ago.
I'm now seeing dp, he too has no children. Has never wanted them. Gets along with my sons, and
Exdp's. He knows that sometimes he'll suggest we do something and I'll decline because I will have ds2 (14) and ex has plans. But if he suggests a meal out, sometimes it will be the two of us, other times for all of us. When he comes over he'll ask do I need anything (bread milk etc), and I guarantee the boys will always suggest pringles, ice cream etc. The boys feel comfortable enough to ask without feeling cheeky, and he asks not because he's buying affection but because it's the norm with us. (I do the same when I pop to see him or his mum for example).
Ds1 (18) actually said to him 'I'm glad you are with my mum, because you make her happy and I can see she makes you happy'. That was one of the nicest things he could have said.
So going back to your question. Children can interfere with relationships. But I seen it more with my friends rather than personal relationships. It takes a certain type of man or woman, to have a relationship with someone who has a child. Because it's way more difficult. Not only to build a relationship with the child, but having the child's other parent and family to, I'm gonna say deal with, but that's not the word I want, I just can't think of the word. But anyway I hope you know what I mean!