What is crossing the line of friendship with the opposite sex when your in a relationship?

I've been dating a my boyfriend for almost 2 years and he's very sweet, caring and does so much for me. The only thing we fight over are his "friends". He had told me in the begining that he has lots of female friends. I too had always had plenty of friends of the opposite sex. The only difference is when Im in a relationship I dont keep in contact those friends regularly. If I see them out we'll talk, but in respect of either relationships I keep a distance. I dont call them, I may email once every 3-6 months to see how their doing and thats it. My boyfriend on the other hand has a few women calling once in awhile, emailing them, texting them.... We'll talk about it and he'll say Im jealous. I trust him but to me its just about respect for your relationship to keep a distance.