People tend to spend a lot of their free time with, and give priority too, their partner, and it seems natural to them to see their friends less. I notice that even I do it. It's just a matter of the extent you do it to. For example, I make sure I still go and stay at my friends house for a few days every now and then, pretty much whenever I'm invited. If a friend is cutting you out completely though it's best to have a word with them coz they might not realise they're doing it, or the impact it has on a friendship. The last thing you want for them is that they will suddenly find themselves alone when the relationship breaks down (if it does), when they will be needing a friend to cry on the most.