Love is silly.
This is not to say that love is laughable, or funny; it's neither. Love is not trivial. It is sublime, life-altering. Love changes.
But love, especially new love, makes us act silly, makes us think differently. We do things out of character for us, we say things we normally wouldn't say and almost immediately the changes begin. The playboy doesn't go out anymore, he only wants one person. The strong woman is weak for her love and cannot stand against it, doesn't want to. When they are apart, each feels the void of the other's absence, and when they are together, they are completed.
Those of us lucky enough to be in love, or that have ever been in love, know the changes it brings to us. When writing about love between characters, the contrast between who they are and who they were can be explored in detail, and can resonate with the reader in a way that mere sex cannot. Sex as a whole is enhanced by love and if a character should have sex with someone else the contrast should be felt. It feels empty, meaningless. It's a form of masturbation.
But get with your lover...the one you truly love...and the sparks fly. Sex transcends physicality and moves into the realm of the spiritual, a connection with another, a synergy. It's become 'making love' and has the power to shift the world.
Real people are usually surprised and may be shocked by the contrast. When writing of a character falling in love, the reader may want to see that difference and remember it. A good way to do this is the way a reader would do it: compare the two experiences and explore them.
Suppose Jack parties a lot with his buddies, goes to bars, picks up a bunch of superficial 'chicks' that he takes home and 'bangs out'. These liaisons mean nothing to Jack. He gains the respect of his friends with his ability to get the women he finds attractive into bed, and demonstrates his power over them by his ability to forget they ever exist. In his way, Jack is a happy man.
And then he meets Jill and falls in love, and his world is changed. He doesn't go out with his friends anymore. They call him 'pussy-whipped' and he doesn't care; his life has become Jill, being with Jill, and after making love to Jill he realizes that he's been a fool. He recognizes that all the women he's loved and left may have hurt like he hurts when Jill leaves him for Bill. He regrets; he will never see his life and his world the same way again.
Or suppose Jill is a successful businesswoman, on her way up; she has no time for sex, or love. She avoids situations that may complicate her life and focuses on her career. She has the occasional lover as a distraction, but afterward he goes home and she goes to bed and the sun will dawn on the same day tomorrow.