It can include sexual harassment but it's not always that. If the person being approached says no, and the asker goes away, then it's not usually called sexual harassment.
It's not always touching.
The question was, why would a gay guy try pick up a straight guy for sex?
I've had straight women hit on me a few times. Every single time my college friend got drunk, she would hit on me....very strange, especially as I wasn't out back then.