Hognet: A girl masturbating doesn't make you a lesbian. Wanting to have relationships and sex with women does.
Countryboyaj: Over masturbation or even having sex too often can lower the sperm count *temporarily*. The body needs time to make more sperm and get up a large enough supply to give them the best chance to impregnate a woman, even though it only takes one. But it only takes a week or two to get the supply back up (I'm not sure on the exact time, but it's not long).
It won't damage his heart or decrease blood flow. It actually increases immune function, decreases stress, and teaches control for the "real thing." Unless masturbation is getting in the way of you wanting to do other things in your life, like study, go out with friends, go out with girls, etc, don't worry about it. It's totally fine.
Less people are having children today for all kinds of reasons. They are waiting longer to have kids than people used to, for one thing. People are not going to be as fertile in their 40s as their 20s. That's only one factor.