The Boyfriend Hypothesis by Susannah Nix

After a string of lousy boyfriends, chemical engineer Penny Popplestone is developing a theory: boys are nothing but trouble! So she swears off men until she can figure out why they keep cheating on her. But her no-men resolution hits a snag when the mysterious and superhumanly hot barista at her favourite coffee shop strikes up a friendship with her.
Penny strives to keep things platonic, but when Caleb gives her the kiss of her life, she realizes he wants to be more than just friends. Tired of always being “good little Penny,” she throws caution to the wind and pursues a no-strings fling with the hottie barista. It’s not like they have anything in common beyond scorching physical chemistry, so what does she have to lose?
Her heart, it turns out. Now, this fanfic-reading, plus-size heroine faces an unsolvable problem. What do you do when being apart is unbearable…but being together is impossible?

An #OppositesAttract Stem Rom Com, book three in the Chemistry Lessons Series, originally published as Advanced Physical Chemistry. Each book in the series features a brand new couple with their own HEA and can be read in any order.