You're entering a tight pussy
or an even tighter ass. Your sliding your cock between a pair
of huge tits. You're stroking yourself off with a sleeve, a
pump, or your old friend Rosie.
-or-
You're stroking your lover's cock. You're sliding a huge
dildo into your pussy. You're greasing up your tits.
You need some lubricant.
Lubes come in various types and flavors, from the down-to-business
water based, condom-safe lubes to the more fun, flavored novelty lubes.
Some have stay-hard properties, while others intensify sexual
pleasures. Any well stocked toy box will have a variety of
lubricants and lotions.