Should I be worried about comments my boyfriend makes on Facebook?

I have been with my boyfriend for 6months & his facebook page is getting us into a bit of a pickle & it’s starting to upset me. He thinks I’m just being childish but should I be worried? I have noticed he is making what I think are sexual comments to other girls & the sort of things he never says to me. He swears that it’s all just a bit of fun & that I should just trust him. I’m beginning to worry that maybe I’m just not good enough for him. He is away at work a lot & these girls are within the area he works. I always thought that if you were in a relationship then you could look but not touch or get explicit with unless discuss with your gf/bf.