The sounds in R18 games are mostly created through a combination of voice acting, foley, and post-production processing, rather than actual sounds occurring on set.
I remember i've watched a video where the voice actor just kiss their hand back palm to make kissing sounds :x
And the background sound effect of body contact/rubbing: Hands rubbing skin; fabric rubbing (clothes, sheets); sounds of creating a "wet" feeling with water or gel.