Am I in the wrong to ask?

Im 21 and my boyfriend who is 22 almost 23 (together 1.5yrs & we are expecting) started a work training program where he met this girl that I actually used to kind of babysit. When I babysat my brothers step kids at 15, this girl was the step kids friend (12), but anyways now shes 18. So he meets her and shes this little attention wh*re who tries too hard to be cute and whatever when guys are around. They talk because they were in the same program for 4 weeks and got different jobs so they dont see eachother anymore. This girl adds him on snapchat, instagram, facebook, the whole bit. Then msgs him on snapchat and says "oh ask the program leader person if we can go early then text me if he says yes" like why text if you can msg on snapchat? Just an excuse to have his number I think. Then she adds me ONLY on snapchat - when Ive known her since she was like 7 - because she thinks I was gonna go try and beat her up (obviously not). It makes me uncomfortable that he rushes to answer her when it takes him literally days to open anything I send him on snapchat, then I guess she threw out the idea of a bonfire after the program and he asks me multiple times kind of begging if "we" can go. I say no because he has a problem of ignoring me for other people when hes drinking and Im not. Im pregnant so I dont wanna go and watch him ignore me for another girl. I get shes prettier and probably funnier than me but Id rather not feel like Im in a competition. He swears up and down that Im the only one he has his eyes on or notices but obviously not true (he saw a picture of 2 girls kissing on insta and tried to act like it was just me being me that made him suddenly wanna take me into bed -_-) I know however he would never cheat but I still have this competition feeling since we've been fighting lots the past few months. My question is, am I in the wrong for asking him to not communicate with this girl because it makes me very uncomfortable and insecure? Or should he do it out of respect?

Side note: Ive never asked him to not talk to somebody even when I dont like them personally. Its just this one particular situation. I dont even think he should be such buddies with someone -- particularly a female -- 5-6 years younger than him.