In some ways "I thought I was sleeping with an eligible bachelor with a view to marriage but in fact he was married and just looking for sex" sounds almost Victorian in its assumptions about men and women, which worries me.
The problem is, while the assumptions have changed, the reality of being a woman in patriarchy has not changed that much.
Nowadays, one can force the married man to pay child support, but it is a battle a woman doesn't knowingly sign up for when she has sex with a man she thinks is interested in a committed relationship. (Look at the contraception failure thread in _Chat before you mention contraception)
Likewise, a woman is not required to have a chaperone nowadays, but when a woman is alone with a man and he rapes her, she will be victim-blamed for being alone with him / drunk / flirty.
I think we would very much profit if a man was legally required to make a statement on his willingness to pay child support prior to sex, and then stick to it. (Lots and lots of men would probably not get sex anymore, but hey, that's not my problem!)