I'm a guy and I distinctly remember all the girls I slept with. Every one of them meant a lot to me at the time.

I know people -mostly women but I'm sure there are both men and women like that (yes in Japan too) - who have slept with lots of people that meant nothing more than a superficial contact for sex, sometimes staying longer with someone if it works out.

What you should figure out before you decide that you want to trust and respect him, is why he slept with so many people, if he hurt any of them intentionally or by accident, if he was maybe hurt, and how he feels about his past. Is his conscience clear, does he feel remorse, is he at peace with his past, is he serious about his future with you?

I think it's okay that this matters to you, but I also think there are many reasons why someone would sleep with lots of people and still be a good person to have a relationship with. Ask yourself what explanation he could give, what he could tell you, to make you feel okay with his past. If it's the mere fact that he's had many partners and that he forgot: that's not going to change; you already know that.