It's practically canon that Newt is a hot piece of ass and everyone wants him (even if he might not be aware of this). Jacob notices and feels insecure because he's not exactly the most conventionally attractive guy. Newt can have anyone, why would he settle for Jacob Kowalski?

Newt is generally oblivious but he notices something is off for this one. After some prying he gets Jacob to share his insecurities. Newt ensures Jacob that he's not going anywhere.

Tender, loving, slow, gentle sex ensues: Top!Newt please.