It's hard to suggest much, because you don't reveal the nature of the problem. It sounds like you knew they were male beforehand, so presumably you didn't like the looks of them for some reason. Maybe you can move past it... maybe you can't... give it a few days or a couple of weeks to see if you can move past it. If you don't want to, or you can't, even with a week or two to adjust.... then tell them... as politely as you possibly can... that you cannot continue. If you can't get over it.... honestly... you don't want to be continuing out of guilt. That's a horrible place to be in.