It's great if you feel happy with having sex with several people. The Q is, is the other person happy with the fact you're doing that, or some other pertinent point, such as the fact the person they are about to have sex with is married, or has an STI or whatever....
It isn't just about you, because sex is a 2 person act and there is another person involved. No-one seem so on think it's okay to withhold key info such as having an STI, the knowledge of which might alter the way the other person behaves or if they chose to have sex with you at all, so why is it okay to withhold other info that might alter their behaviour or if they have sex with you at all. And no, I don't think the onus is purely on them to ask.....there are all kinds of scenarios which someone might not be happy with and which could hurt them and they cannot possibly list all of them, the the person who has the STI or who has had sex with someone else that day already, or who is married to someone else knows...so the onus is on them to say and allow someone to choose in receipt of pertinent info. It may well be that they don't care and continue...or it might mean they don't. Shouldn't they have that choice?
How many people have been hurt by someone having an STI and not telling them? No-one thinks it's okay. How many have been hurt by getting involved with someone who was married when they had no idea they were married as that info was withheld, or by someone they thought was the start of a genuine relationship, but then wasn't? Lack of communication has caused hurt and the people saying they have no responsibility to communicate anything at all to a stranger, have caused that hurt.
I think at the point where you have sex, the onus is on you to recognise the scope for great pleasure but also hurt too and that by having sex you have to take some responsibility for that person. It might be very brief and limited if it's literally a one night stand. It might be using a condom or saying something about an STI, but it should equally be about something like being married or only interested in a one night stand or something casual. It doesn't have to be a deep revealing of the soul to an almost stranger, but an acknowledgement that they are a human and that in order to engage in sexual activity, people need to know it's safe for them physically and emotionally.
I think if you deny other people that info, you essentially say they can deny you that vital info too - and that in itself does suggest a low value placed upon yourself. I would be really disappointed and probably hurt if someone I met wanted to have sex with me and didn't tell me they had an STI, because they thought I wasn't owed that info and it was up to me to ask. I'd equally be upset if they were married and didn't want to tel me that info upfront, or that they wanted it to be extremely casual - I wouldn't need to know they had sex with someone else earlier in the day, simply that they saw what was happening with us in the context of no commitment or future plans at all. Then I could decide if I was happy with that.
I expect that basic level of communication from a fellow human I plan to have sex with, even if I will never see them again or even tell them my name. It's because I value myself and think I'm entitled to that so I can protect myself. And therefore, I think they are entitled to that info too and for me to be forthcoming like an adult about anything vital which might alter their choice. And I'd have to accept that in receipt of that info they might not want to continue to have sex with me....but that has to be their choice based on info, not mine to withhold.
So ultimately, I think refusal to be willing to acknowledge the other person as someone with feelings and some basic rights to information, isn't about having sexual liberation and freedom and enjoying no strings sex and being freed from puritanical patriarchal views, it's about men or women who do it, not having any respect for the other person and also showing a lack of respect for themselves....becaue they cannot expect others to provide them with that basic info either, if they don't feel any sense of an onus to provide it to others.
It's not about not having sex with lots of people. It's about the communication of basic information about yourself that allows people to make informed choices.