If he doesn't want to have sex with that person, it means he has already done it with him.
He doesn't do the same person twice.
but the rumors say so! we don't know if it's actually true!
No. He's referring to the 'doesn't shit where he eats' principle. Meaning he believes the reason he doesn't want to get involved with a colleague, is probably because it has ended badly for him previously. Obviously this is his own speculation, based on the fact that the guy being gay is an open secret amoung the men.
But basically, he's assuming since he has a rule in place there must be precedent. That's all.
The fact that he "doesn't want" to means he "did it" before.
Can anyone tell me what does that mean??