I believe you should never have sex with a "hidden agenda".
You should only have sex because YOU want to. Those who do otherwise and find their relationship does not improve or possibly ends often walk away feeling "used". (Expecting) anything beyond an orgasm can lead to disappointment. Having sex can be an expression of love but it does not mean you will get more love. You can have sex with someone you don't love and you can love someone you've never had sex with. Never confuse the two. Some people believe having children will strengthen their marriage. Love and affection are given "freely". One should not have to "do anything" to be loved.
“While we are free to choose our actions, we are not free to choose the consequences of our actions” – Stephen R. Covey