The idea of people choosing their significant other over their friends is a tough subject for me to understand. No matter how hard I try, I cannot fathom how anybody could throw away friends, especially long term ones for something that could end up being temporary.
Unfortunately, if your significant other is not friends with your previous friends, or does not get along with them, it becomes hard to maintain both the new relationship and your old friendships. Even though it is unfair to the person who is stuck in the middle, sometimes the ultimatum is unavoidable.
Most people are able to find a balance between their friends and their romantic partners. This is hard to do but those who value both their friends and love lives do their best to find the best of both worlds. Yet, there are still those people who insist on disposing their friends and replacing them with a significant other.
Some people who do this might argue that they surround themselves with friends to fill the void of not having a significant other in their lives. As a result, they dispose of these friends when they find a romantic partner. Friendships should be mutual and it's morally wrong to rely on somebody for your own convenience.
Friends are not by any means disposable and should not be seen as temporary placeholders. Personally, when I become friends with somebody I am there for them when they need me and I expect the same in return.
Treating people this way hurt them more than one might think and will undoubtedly end up hurting everybody involved in the long run. With this in mind, I think people with this mindset tend to forget that if the relationships come to an end, they will not have their friends to comfort them.
Friendships tend to be much stronger, and generally longstanding, whereas romantic relationships tend to weaken and end up being short term. Sure, relationships and friendships are always different given those involved and their respective time frames, but friendships should still not go undervalued.
A relationship should change you for the better, not for the worse. If your significant other is telling you who to spend time with and how to live your life, maybe they aren't the one for you. A reasonable romantic partner would understand that friendships are important too.
In the end, choosing between friends and a significant other is a personal choice and varies by situation. However, people tend to forget how important their friends really are. Remember, relationships come and go, but friends are forever.