I’m sure your boyfriend has emotional feelings for you, but he’s carried away with his feelings about sex, so that’s what he’s pursuing in these other girls. The younger girls also look up to him more because he’s older than them. At 13, 14 and 15, the age gap between their years and an 18 year old boy’s seems a lot greater than two people with that same age gap in their 20s or 30s or 40s. An 18 year old boy usually has a lot more life experience and sexual desire than most girls of 13, 14 and 15, so he’s exotic and interesting to the younger girls, and for a lot of girls who are insecure, he’s a status symbol.
I’m sure he loves you — otherwise he wouldn’t have introduced you to his family and taken you on the dates he did — but his sexual desire is what is driving him right now, and he doesn’t think of you as someone to experiment with and move on from. Take it as a back handed compliment. Random sex with strangers IS a lot easier for him than a real relationship and it’s all he’s really after right now.
Eventually, that will change. But for now….his behavior falls in the range of normal.
I hope that helps you understand your situation better.
Please follow me @AskAprilcom on Twitter.