Why do guys like putting their fingers in girls mouths?

I'm assuming you've experience this during sex or making out and not just randomly walking down the street. If my assumption is right, I'd guess it's probably the same reason why they get a little turned on watching girls lick or suck a popsicle or lollipop. Guys are very visual and watching girls slowly lick, suck or kiss something turns them on. Maybe it reminds them of blow jobs or kissing. Putting their fingers in girls mouths lets them watch and feel at the same time... even better than popsicle watching... haha.

BUT if you're not it the mood, don't like it at all or just don't get it, then there's no reason you can't kindly and lovingly ask him not to do that anymore. Then maybe tell him something that you do like to do...