The protagonist began dating a femboy.
When he found out that the femboy had never even masturbated, he decided to teach him about sex through videos and other means.
However, when he saw the femboy getting embarrassed watching a pornographic video, he found himself becoming aroused.

Thinking about doing something to the femboy, the protagonist skillfully convinced him to wear women's underwear and laid him on the bed.
It's only natural for a lover... he said, and began to caress his body.
At first the femboy was flustered, but it seems he had masochistic tendencies, and the more he was teased, the more seductively he began to moan.

And finally...

[From Getchu]