My advice is not to get too hung up on describing the act. Make it clear what's going where but try and avoid turning the scene into an anatomy lesson. Try and give the scene some kind of emotional hook. Use it tell us something about the charecters. How do they feel about what's happening? How will it affect them going forward?
We have a thread with tips for writing sex scenes: https://forum.chyoa.com/threads/tips-for-writing-sex-scenes.1958/