I'm a 25 year old male and I suck my thumb almost daily. I've also struggled with a lot of the feelings you're struggling with, such as feeling like it's worse if you're a guy and it's some how cute when a girl does it as well as the other things you mentioned. I've had a girl laugh in my face... I've also had several people not care at all. Anyone who really cares about you won't mind and if they do mind, they obviously don't care. If you think about it, it's really not a big deal. If they're not willing to accept you over such a small thing, that's their problem and they're the ones who are childish. I didn't always feel this way. It took me a long time to start feeling comfortable with it and with myself. I promise there will be people who will accept it and not care. For the record, it doesn't make you childish or any less manly. It doesn't. There plenty of of men here who suck their thumb and there are plenty of women here who are with guys that will tell you the same thing. It's not anywhere near as uncommon as most people think. Best of luck to you.