Why does he pend so much trying o convince me he's not gay?

I met these two bi ual guys and we had a 3 some. It was supposed to be this 1 time NSA experience, but I have struck a friendship w/ 1 of hem. We talk, and the first thing he said to me is.."you know i'm not bi ual"...which was a suprise to me b/c I SAW him having oral w/ a guy. He then goes on to tell me that he likes to wear women's underwear, which I saw pics of, but he still tries to convince me that he's NOT gay or bi, but straight. I have no prob w/ this, but apparently he does, b/c he always makes it a point to tell me he's not gay. Why does he he spend so much of our conversation to prove he's not gay, when OBVIOUSLY, i'm cool either way?