You’re not overreacting. Your instincts are correct. There is absolutely nothing wrong with your boyfriend spending time with the guys, but hanging out at nightclubs on weekends where drinking, dancing and hooking up are the activities that most people gather for, he’s skirting temptation too often — or else playing at being single.
Since this is new behavior after your being together for a year and a half, and talking about a serious future together, I think he’s scared of commitment and losing his bachelorhood. The year and a half mark during a serious romance usually means the guy needs to step things up and decide if he wants to marry your or not. In fact you should be thinking about the same thing at the same time.
Whatever you do, this shouldn’t be the cause for you becoming “his mother” — in other words, don’t fall into the role of telling him what he can and cannot do. That is poison for a relationship. You may express your disappointment and concern that he’s not that serious about you if he’s going out to nightclubs on weekends without you on a regular basis. He may hear your concern and invite you to come along. Or he may tell you that he’s really not ready to commit to a relationship.
But even if he doesn’t say it, and he doesn’t change his nightclub ways, then you have to really decide if this is a guy who is able to commit to a future with you. Usually, a man who is ready to get married starts acting married. He’ll hang out with other married couples. He’ll do things with you and his friends. He’ll want to take you out on Saturday night.
If he’s starting to shed his serious boyfriend ways, he may be unwilling to take the next step with you, but is unable to express this to you in any other way.