I'll preface this by saying: I have very little sex drive, so take it with a grain of salt, and I'm bi with a heavy preference for males, so not entirely gay.
Personally, the sight of a penis does not arouse me, but the person it is attached to might (Though, as I said before, very little sex drive, so probably not).
People can be aroused by their own bodies. One of my straight girl friends used to (idk if she still does) masturbate while looking at herself in a mirror.