It isn't about him catching it. When a person had mono, their spleen enlarges, and too much activity (i.e. sex) can cause it to rupture, causing internal bleeding and possibly death.
Actually, this hasn't been proven at all.
NO, I am NOT a doctor. But I did stay at a holiday inn express last night. Oh, and a case of mono and the resulting enlarged spleen almost deprived me of a tier 1 ~$200,000 ROTC scholarship.
The spleen may be mildly enlarged by mono, however, there is not necessarily any link between enlargement, activity, and rupture. The assumption is, that since the spleen is enlarged it is weaker. If it is weaker, it must be more likely to rupture with activity. However, statistically, more spleens are ruptured sitting on the toilet than during any other action. There is no proof that the spleen is weaker with enlargement, and the likeliness to rupture wouldn't increase with sex, unless part of your sex is punching your partner in the abdomen.
Just some public information I spent months studying with my MD to fight "the man" holding me down
