Have you explained why and how did you intercourse with someone?
In this situation, usually it is hard for MALE to accept.
Honestly, the reason most of them can't accept and never admit is they're selfish.
Nobody is willing to share much of their own 'things'.
It did happened commonly especially those who just started in a relationship will easily lose control and intercourse.
What if they break 1 day? Usually male will never consider this question because most of them don't care much.

There are many cases similar to yours nowadays.
You should really seek for his understanding that your past is PAST. Everyone made mistake and it does take very long time for a guy who really loves you to accept.

If you think that he is the right one you are seeking for, you should be very patient in making him understand and deepen the feelings between you and him.

Cheer up and good luck in your relationship.