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.