You should be happy for her because she was enjoying the natural act of sexual pleasure. We are all human and we have the natural instinct to engage in sexual intercourse when we reach adulthood.
Some sexually active mature mothers consider an affair with their sons as “family entertainment” that transcends the barrier of “blo*od relatives having unprotected sex. I cannot condemn any person who engages in activities like this because sex is a natural act, and lust will always be satisfied regardless of family ties.
Your comment about your wife benefiting from a young cock is very appropriate for any husband who shares his wife. I must admit that I’m in the “same boat” as you except for the fact that my wife is young and ready for “action” all of the time.