Most Helpful Guy
Love is about making sacrifices for the other person. If they can't even sacrifice having sex with someone else then I would say that generally speaking they don't love the other person very much. They may enjoy your company and miss you if you go, but that isn't the same as love.
Even if they did love them, they clearly can't ever be trusted or respected the same way again. Trust and respect are at least as important as love in any relationship.