I see ads for a program that draws pictures of women in sexual situations but boy when penises get involved it turns into nightmare fuel (disembodied, decapitated, bifurcated, pointing in the wrong direction) which makes me glad I don't have castration anxiety.

The really obvious MR sex app is one that overwrites your partner with somebody else like Strea or Bremington. Not sure if most partners would like that. You might get something that feels like motion sickness if there is too much of a difference in breast size.