@Medusax,
If a woman wants a relationship... whether it is a fairly short sexual encounter (i.e. "a man to spread his legs for her") or something with the possibility for a longer term meaningful relationship... what would you suggest she do?
Asking someone you find sexually attractive "can I buy you a drink?" is part of our culture, it is a ritual part of flirting which may or may not lead to sex. It is probably preferable to just asking someone "do you want to have sex?" (at least I have found that the former is more likely to lead to a sexual encounter). Generally casual sex starts with flirting, there is a game that is played by both sides, and then it ends up with sex that is desired and hopefully enjoyed by both people.
There are long term relationships too that start with a "can I buy you a drink". Buying a drink doesn't mean anything other than possibility. The way forward is then negotiated by both people and many time it just ends with a drink. But refusing a drink generally shuts down any possibility of any interaction.
I assume that you agree that sometimes sex, even casual sex, is a good thing for many of us. For people who don't want this... of course refusing any interaction is reasonable.
Many of us like sex.