But frankly there are worse things to be mistaken for.
Oh, yes. I'm bis
exual, and my best friend V is a straight woman who participates in the local LGBT Pride march with me every year (as a lot of straight people do to show their support).
A couple of years ago, we were striding down the main road on this march with linked arms and a placard each. We passed the pub where I worked then, and say a lot of the old regulars at the wondows. Several of whom saw me, looked startled, and then began nodding and smiling in a
terribly understanding manner and putting their thumbs up at me...
On the front of the signs were some general g*a*y-rights phrases. On the back, we then realised, V's said "OUR LOVE IS NO LONGER CRIMINAL". Ah...
I said to her, teasingly, "They'll all be asking about my secret girlfriend at work on Monday!"
She replied, "I'm impressed they think I could pull you!"
Bless her

ETA: I'm not complaining that a group of customers who frequented my workplace were showing that they were trying to be nice. But the whole "It's OK, I still like you" and 'being terribly understanding' can grate a bit at times... (And yes, I know that most of the time it comes from the best of intentions, so I just smile!).
freakyfemme, I agree with you it does sound like this guy is also acting with the best of intentions (or so he thinks!). However, he is being entirely inappropriate, getting ruder, and this is essentially harassment - to incessantly bang on (no pun intended!) about another individual's sexuality is horrendously annoying at best, and threatening at worst.
Do you know his own sexual orientation? (I ask as, if he keeps openly talking about the whole g
ay-Straight Alliance thing, then it is likely he is also open about being g
ay/bi/straight).
If so, could you try asking him quietly how he would feel if you kept trying to convince him that he wasn't REALLY g
ay/straight, he was fooling himself, he simply MUST actually be straight/g
ay because he...[and then make up some really ridiculously overblown, stereotypical examples of gayness/straightness, which display how daft his own words to you are].
If this doesn't work, you will have to be very firm and stern and sharp with him, but without sounding defensive (though you have every right to be...but he would probably take that as confirmation you were still "hiding" it [sigh]).
Good luck!

P.S. I sing tenor in my choir (officially) and I'm only "halfway g
ay", so to speak, myself, so I fail to see what the heck singing alto has to do with it! And one of our completely straight male basses can do a very nice alto...