That's not quite it, no.

It DOESN'T sound like slang and that's the problem. Because the rest of the story is a lot more similar to the way a teenaged guy would think and talk. Then suddenly he's using some really old fashioned terms? There's no way a modern guy would talk like that unless he was partially joking at least.

If you want to make his sound serious. I'd say stick with something like:
"I'm a GUY! A very available guy! Do I -look- like I'm gay?!"


As for 'Proper fic': Whether it's slangy or not doesn't matter as long as it works right. I don't see street gang members talking or acting like they're college professors... so in relation to that, it'd be logical to use more common language. It's what works that counts and makes it 'proper' not the use of fancy language.