I typically become less social when in a relationship. When living alone I'll jump at the chance to go out with friends, but in a relationship it seems far less appealing to go out.
The question is whether that's less social. Maybe less externally social, but if that's the case it seems like it's because your significant other is fulfilling your need for social interaction. I don't think there's anything wrong with that, really.
This is exactly right. I actually became more social when I got married, but not because I'm going out anywhere, just I now have a built-in, zero friction way to be social.