Please help me! boyfriend confusion...?

ok so my boyfriend has about 10 friends that are girls and 4 guy friends. and sometimes he gets invited over to the girls' houses or movies and stuff and like 3 different girls will be there. it pisses me off that i have to find out on facebook and he never tells me. i trust him with my life and he is very dependable but this just annoys and worries me sometimes. what should i do??