Bisexual

So, I'm bisexual and I have always wanted to have sex with a girl, but the most I've done is kissing. I really want to do more but my boyfriend won't let me - not even a threesome. He says threesomes make him feel like it's cheating, which I do kinda understand, but how do I explain to him that I really want to do something more with a girl before it's like...too late. While we are young and before we are married etc.