Allow me to explain why such an offer is not OK: all women have had at least one experience of turning down a man, and receiving threats, insults, or violence.

Allow me to explain why such an offer is not OK: all women have had at least one experience of turning down a man, and receiving threats, insults, or violence. That's especially true when the offer is explicitly sexual in an otherwise non-sexual context. You may know that you intended to take her declining with good humor, but she does not unless you are already a friend. Even if retaliation is not immediate, it may happen later. So the offer itself makes her apprehensive. Men are much less likely to be the target of that kind of assault by women, so you can be pleased with such an offer. But to give you a sense of it, would you be so pleased if the offer of sex came from a homosexual man? The analogy is imperfect, as all analogies are. You may be bisexual, and thrilled with an offer like that. But you can inquire with straight friends, many of whom would feel threatened to have an offer of sex from a male stranger.