Don't assume that he wants to do it again, or that he doesn't, or that it means a lot to him or nothing. Without asking, you can't tell. I suggest you say "Can we talk?" and in a safe environment, such as a cafe, you say "About the other night. That was a one-off for me, and I'd like to put it behind us." And listen to what he has to say. If he's a jerk who sees you as someone for sex and nothing else, and won't stop hitting on you, treat him like any other jerk.
One risk you now face is blackmail, if you value your reputation as straight. Especially nasty would be if he threatens to tell everyone about the encounter if you won't have sex again. My advice in that case would be to call his bluff and say "Do your worst." What would he be telling them about you that would be worse than he was saying about himself? The people who don't care that he's gay won't care if (they think) you are, and the 'phobes will hate him first.
If he's not a jerk, he'll say "Sure, it was nice while it lasted. I could tell you weren't really into it," and you can even be friends.
As for giving blood, I'd take the Clinton Amendment. If you didn't do anything that put you at risk of HIV (receptive and/or unprotected), then for their purposes, you didn't have sex. The blanket ban on MSMs giving blood looks more and more like discrimination as the heterosexual epidemic increases and testing improves.
