Not allowing Lena to cheat on Ian with Jeremy would be a huge missed opportunity.
Look at Cindy's route. Cindy is the most popular female NPC, she's attractive and her scenes are hot. But her route wouldn't be as spicy if she wasn't Wade's girlfriend. This is part of what makes her so popular and tempting. She's Ian's forbidden fruit.
Cheating, corruption and BBC are popular kinks. Jeremy is Ian's friend and Louise's "boyfriend". He's also the most popular male NPC.
If we play as a seemingly good girl Lena who falls in love with a loyal loving Ian and commits to a relationship with him, but at the same time struggles to suppress her lustful fantasies, cheating with Jeremy behind Ian's back would be the ultimate sin