Gosh, the judgement in some of these posts...
OP, you're not married, you're not in a relationship... if you want to have sex, and if you want that sex to be ONSs, then that's up to you. I wouldn't enjoy ONSs but equally, I've never been divorced, and I can understand why going through that would make you want to do this. I might well feel the same in your shoes, who knows?
To answer your question, while it's both your responsibility, let's face it, women are more likely to get stuck with a bad outcome.
An unwanted baby? The man could just disappear. Unless you've seen their passports, run a search on the number and verified they are who they say they are, you don't know if you've got their correct legal name. So, the hardship of raising a child, the cost of raising a child... that's on you. That's part of the joy of being female. We carry the children, so we can't just opt out and run away. We can get abortions (in this country, at least), but that's still time, and cost and emotional labour that they don't have to deal with if so inclined.
STIs? Well, again, let's face it - he's more likely to have them than you.
Most women do not want babies from ONSs and will take precautions involving a condom - possibly a condom and a pill. So, if one of these men convinces you to have to have sex without a condom, it's more likely you'll have had safe sex up until that point and be disease-free. He'll be the irresistible hunk who has made you change everything for him.
Many men want to have sex without a condom. They're more likely to try to persuade their partners to have sex without a condom. Statistically, that means they're more likely to have more unsafe sex than you.
Fundamentally, it might be an equal responsibility but it's not an equal consequence and it's not an equal risk. That's why some of them don't.
The thing is, not all condoms are the same size. It makes more sense that the man buys and brings them. It gives him more control over the baby dilemma and also means he's more likely to pick a good fit and not experience a condom mishap. It's just tightness and arrogance to not bring condoms. What makes him think he'll be so hot that you just have to have him, with or without protection?
I'd consider not sleeping with the ones who don't bring condoms to a hook-up. As you said, the one who was the best brought his own. I think you can tell a lot about how generous or stingy a man is going to be in bed about whether he bothers to buy and bring condoms with him!