They were dating casually and flirting, I'm not getting a vibe that he told her he was her soulmate. Maybe he just wanted to hook up for sex, like many men and women do. Sounds to me like that was the situation at least, and he couldn't get over the fact he wasn't aroused by her breasts. Young guys fuck up and handle situations badly, because they are inexperienced. Without being aroused sex was out of the question, and he tried to leave without saying why. He couldn't come up with a good lie on the spot and just told OP the truth about how he felt (note that I'm not saying the truth about her boobs, that's just his opinion). Aren't we always telling young people it's fine to back out from sex if they change their minds? He changed his mind, and backed out. He didn't say it was because of her breasts, she pushed him and he obviously fucked up. If he didn't care he would have said it outright, but he said it and what was he going to do, stick around after that?

I think it's cool that people are supporting OP and it's right that she shouldn't worry about her breasts, many guys don't give a shit just like many women don't care that some men have odd/small penises or beer bellies or whatever else. But some do, and they are not necessarily bad people. On both sides. I'm getting a bit of a vibe in her like when men get turned down by women and start saying "well she's a bitch anyway". No man, you just didn't meet what she wants. Works both ways.

Just saying, don't condemn the 20 year old because he fucked up through inexperience. I've heard enough first hand tales of people getting too far, changing their minds and in the panic saying some fucking dumb things. You're there in bed, possible naked, you don't want to have sex because of your partner but you don't want to tell them. Some people blurt out dumb shit, which includes being overly blunt at times.

If he had laughed or told his friends or anything like that, then yeah, fuck him.