To go along with Lex...
No question: Dave the Barbarian
A big, muscular young guy who loves arts, crafts, cooking and decorating, and abhors physical violence. It's possible the writers simply wanted to paint Dave as the "anti-Barbarian", but even that sort of fits in the with the undercurrent gay theme. He's "got the equipment", if you will (a sword, the muscular body, the lineage) which everybody assumes means he'll be one way...but he'd rather "be himself".
One of the later episodes gave him a girlfriend. This fooled nobody. Not even the girlfriend.
Likely, but by no means certain: Don Karnage
The main nemesis in the Talespin cartoon, he was the head of the gang of "air pirates". All the evidence was rather loose.
* He lived on an island with all male pirates.
* Whenever a female came into their midst, the other pirates would pant and drool. Don Karnage never seemed interested.
* He was excessively vain. He cared more about looking good committing his acts of piracy than the acts of piracy themselves.
He had an odd mish-mash of an accent, too - French-Spanish-Italian - and a fun way of mangling the English language. ("You juvenile delinkity-wink!") Quite a few viewers have said they thought he could be gay, but straight female fans rather adamantly want him on their team.
A decent shot: Donatello.
The "oh, right, that OTHER ninja turtle" ninja turtle. The quiet, geeky one who probably would rather stay home and read. There's really not much evidence here - "no interest in the opposite sex" is about it - but again, it's surprising how many people have said they thought he might be gay. Perhaps in response to this, in the most recent incarnation of the cartoon, they turned Donatello into the one who's always ogling the women. Which seems to have had the opposite effect - most long-time fans consider this "overcompensation".
Unlikely: Michelangelo
A few folks paint Michelangelo as bisexual. He's clearly not strictly gay - he's always seemed interested in women, at the very least. It seems the bisexual tag comes from the fact that he "likes to party". His easy-going persona seems to indicate he probably wouldn't mind going to bed with a guy. But that's pretty much all you get there.
Lex