While I largely agree that the scenario of Lena liking Stan is fairly unlikely, there are a couple of possibilities...There is porn logic (a MC attracts all female just by existing) and there is no logic (Lena being attracted to Stan)
Lena interacts with Stan and finds that they share a mutual love of photography (albeit at opposite ends of the lens), option one she gets to know Stan and feels sorry for him (Lena can be very open hearted because she has been badly treated in her past) or while shooting, she gets a thrill from commanding and dominating him. Are either of these scenarios strong foundations for a stable relationship? HELL NO! although I could certainly see Stan making a decent sub if Lena goes all dominatrix on him, and heck it could be satisfying/gratifying for both parties.
Having Stan be a pity relationship is potentially extremely destructive, but with guidance (and a whole lot of work) he could be good for Lena, rather than constantly cheated on by muscle packed pretty boys who do nothing but abuse her.