It isn’t sexy, but it will save your mattress. Chux pads.
They sell disposable ones, but I don’t care for those at all. They tend to tear or just not hold much liquid. These are washable and stay where you put them for the most part. If you know you’re going to be having adult fun, just put one down and have fun! Toss in the wash and voila, good to go. The link I posted is for 4, for less than $30.