Sexual compatibility is entirely subjective to the two people involved. If they are not compatible then it makes sense to end it.

There's a whole chapter of the Kama Sutra about compatibility between people based on their genitalia sizes (here's a short summary). I'm not saying the Sutras are right, but it might give some perspective to notice that people around the world have been aware of the importance of sexual compatibility for thousands of years.

Long story short: men and women come with different sizes of junk and compatibility does matter. There's someone for everyone, but not anyone can be with anyone - even guys with huge cocks get turned down for being too big sometimes, too.