What is the Catholic Church position on sex for pleasure or the infertile engaging in sex?

Sex in marriage is a gift from God. Paying for sex is a sin that degrades both the buyer and the person trafficked (as well as society). Cohabitation is prohibited.

Each sexual act is not required to be intended to produce new life. A married couple may not licitly refuse the possibility of new life in their marriage. Artificial means of regulating fertility are proscribed.

Subject to this, the church in a sense says enjoy or perhaps even Mazel Tov (not Molotov) to the sexual acts of couples you reference.

Sex for pleasure = sin.

Infertile = God does't want you to have children.

You are basically one sin on top of another so go ahead and do it.

Although in science sex is pleasurable to encourage humans to reproduce.

Sex was designed by God to unite the man and his wife and from that union, the child can be born into a covenanted safe environment. If there are no children--that is God's choice.

"...or the infertile engaging in sex"

I think that qualifies as sodomy.