Most people get more out of a relationship than sex.
And on top of that, a lot of problems that were before solved by having a partner (something breaking, don't know how to cook, just lonely and want to chat, etc) now can be solved by various businesses and services.
I think that it's possible that for some people classical relationships aren't as optimal anymore.
For one thing, there's the validation that someone is interested in having sex with you in the first place.