I actually posted a similar question here a while back with both positive and negative feedback. The difference with my situation is that I was already in a relationship with the guy. Too be fair, he did give me full disclosure of his incredibly active sexual past before dating. He knew that I'd slept with under 10 guys, and he figured his past would either turn me off or be a constant worry for me. I liked him so much that I said I didn't care about his past, and we became a serious couple. I should also note that we'd known each other for a year before dating and we never had sex until after we became a couple. 
I'm completely head over heels in love with him and the feelings seem to be mutual. He's without a doubt the best thing that's ever happened to me and says the same about me for him. While I'd love to say that his past is just a thing of the past, unfortunately, it's still something that causes an issue at times. 
Sometimes, it's caused by my own insecurities. For example, if we only have sex a couple times a week, I start to wonder if he's grown bored with me or if he's lost sexual attraction to me. I'm like...he had sex so often in the past, why isn't he having sex that often with me? Or I'll sometimes be having sex with him and imagine all the guys who have seen him in these positions. Or sometimes I wonder if he misses sex with other guys. I even wonder if he thinks about guys who were better at sex than I am. For months I worried that he was cheating on me, because he was a bottom in 99% of his past sexual encounters, but in the 9 months we've been dating, he only wants to top me. So, I wondered if he was bottoming for someone else. 
I think the worst is that every time I move past his past and forget about it, we hang out with a friend of his who brings it up, or jokes about how they can't believe he's monogamous now, or they'll say something jokingly that actually offends me. Then all the worrying comes back again. I'd be lying if I said I haven't lost sleep over this issue. It's even been the cause of a few arguments. 
In the end, it's really up to you. With the exception of his past, my boyfriend makes me the happiest man on the planet. He tells me that he's always wanted a monogamous relationship, but never found the right guy. So all I can do is believe him. I take pride in the fact that out of the hundreds of guys he slept with, no one intrigued him enough to have him wanting more than just a sexual relationship (and remember, we hadn't even had sex!). Yes, the past still presents a problem even 9 years in, but I want him. I want him for the rest of my life. If it takes years for me to get over his past, then so be it. In the end, he's mine.