Would it be OK if me and my ex started having sex. Just so we can both get off. Is that wrong or what?

Nope. As long as it's a mutually just sex, and no one gets feelings again. (Ever heard the saying "nothing like sex with the ex"?)

Naw nothing wrong with that as long as no ones getting hurt lol

It depends on if one of you may still have feelings for the other, that aren't mutual. If its just sex... and you both agree on that, then go right on ahead. But if there's even the slightest indications of actual feelings, I'd say drop it-just because it makes it harder to walk away later.