I think it's wrong to marry this girl if you're still having misgivings about doing so because it would make you uncool for not having many sexual experiences. As long as you see merits within promiscuous behaviours, you should explore and resolve them. This does not mean acting out on them (though you can) but at least thinking about it more. About what you value. About what the input of others and society's themes mean to you. Are they right? Are you right?
Marriage is supposed to be a serious commitment, you need to be content and resolved about it or it will be troublesome to sustain it, and it would do a disservice to whoever you'd marry. After all, consider if your girlfriend had similar misgivings. She'd do you a disservice to marry you if she would be wondering sometimes "perhaps I should have slept with a dozen men before marrying this guy".
There is also the question of exclusivity within marriage as some couples have more open policies regarding other persons.
One valuable thing is really just to talk about this. Relationships should be open, you should be able to discuss your feelings.