Sex, relationships, and loneliness are all distinct domains. There's important overlap to be sure. But they are not the same things. You can be a casanova and still be lonely. You can be stuck in a bad marriage and feel very lonely indeed.
I think a lot of modern people (regardless of gender identity) have problems making and keeping friends, too. Long-term friendships take work, sacrifice, and compromise. It's not just compatible interests and personalities.