No girlfriend at the moment either, but if I did have one and she wore a belly shirt, I take it what you mean is how would I react to other guys checking her out. Well, so long as they only look and don't touch, I don't mind. Plus, if she's walking around in a belly shirt, that would make it all the easier to poke and tickle her bellybutton every now and then. In fact, why wouldn't a bellybutton fetishist want his girlfriend in a belly shirt at all times? That would be a dream come true for most, I would imagine.