Obviously a lot of people here disagree. But I think for most people the answer is unequivocally yes.
I guess if you think the bare minimum for friendship is someone you say "Hi" to at the water cooler every once in a while then no, you can have lots of friends aside from your partner. But for many people your friendship/relationship with your partner consumes most of your time (outside of work). Add on to that kids, and that's a wrap.
For most people that replaces the majority of the strong friendships that might have had when they were younger. Sure maybe they still have a couple of friends, hence the "less".
Actually now that I wrote that I realize the answer is no. People consolidate their friendships partly due to maturity, but also in large part because they find friendship/companionship in their partner. People don't get partners because they have less friends, they have less friends in part because they get a partner.