It's bad to lie about using condoms.
Dan is about to have sex with Jen, but she wants him to wear a condom before doing it.
Dan wants to have sex.
Dan puts on the condom, because he thinks it's the responsible thing to do.
Dan pretends to put a condom on and then penetrates Jen.
Jen is put at ease as they have sex, and she doesn't have to worry about anything.
Dan is putting Jen at risk of becoming pregnant when she doesn't want to be.