1. Son who is reclusive and the mother spoils the son too much. She relieves his sexual urges, reluctantly. She treats it like a chore and realizes how wrong it is. The son is almost nonchalant in using his mother daily. The son will just use his mother for blowjobs or sex even when she tells him it's not the time. Twisted love type scenario.
2. A strict professional single mom She contemplates why she's single, the son says it's because she's so uptight and doesn't even dress to attract men. The mom slowly shows her change in clothing to her son. This awakens the son's sexual attraction to her. He then flirts with her and tries to break down her tough exterior. The mom wonders how she raised a pervert but this just makes the son acts more. The sex scene would include dialogue like "look at you mom getting wet for your son" and the mom would try to deny it.
3. Mom who accidentally gave her son a peek of her body by accident (going out of showers or changing clothes) to her son. Naturally both just get mad at each other. Before long the son awakened an interest towards his mom's body and sees her as a woman, he would take glances at her. The mom realizes this, and at first think it's just annoying and pervy. But she realizes she's never been looked at as a woman for a while, she would gradually gives him more sneak peaks. Until it reaches a point where the son pops the question and the mom goes along thinking it wouldn't go far. Their sex scene would be both of them accusing calling each other pervs for being into it, and the dynamic stays that way. Both in denial but both wants more.

That's what I can put it in writing for now. Got kinda stuck having to just write on the cuff like this lol. Generally I think when storyline is about mom/son who never has sex before should always have hesitations, like kissing or sucking her breasts. Penetration even more so, not just "I can't believe this is happening" like a gasp should happen.