So this game is about incest right? (SPOILERS)

[ⒹⓄⒼⒺ ♔king of le maymays♔]

First of all you can tell that Thomas is adopted. In the family portrait, the mother, father, and Gabby all have black hair, which is a dominant phenotype (expressed gene, for all you people who failed biology). Since it is impossible for a blonde son to come from two black haired parents, he was obviously adopted. (NOTE: there are SOME genetic workarounds this, but require statistically improbable assumptions of mixed heritage)

Secondly a common image in the game is Gabby being "stabbed". The knife is obviously a phallus shaped instrument, and Thomas is shown "plunging" it into Gabby, a metaphor for sex. She bleeds in the very first scene we see this, signifying that she lost her virginity to Thomas. Further evidence of their relationship together is in the Final Descent ending where Thomas dreams that they even had a chance to be married. After some episodes of incest (not 100% incest I suppose, since Gabby and Thomas are not related by blood), Thomas's foster parents catch him in the act, and get him institutionalized at an asylum either by bribing the asylum to take him away (the family seems wealthy) or by construing Thomas's incest as some mental disorder.

While Thomas is in the asylum, Gabby kills herself. You can tell this occurred by the numerous "hallucinations" of Gabby's death in game: Falling to her death, death by hanging, ingesting herself (in the Final Descent path, you can find Gabby eating her own body -- destroying oneself, a metaphor for suicide). This is probably because she was truly in love with Thomas. Thomas receives news of this while in the asylum and ends his own life as well likely by finding a sharp instrument and cutting his wrists. You can find evidence of this in some cutscenes, where Thomas is splitting his arm open, stabbing himself, thrusting his hand into a meat grinder while laughing (a sign he has gone insane from the loss of Gabby and welcomes his death) or in one of the hallucinatory "endings" where Thomas wakes up in a bed, but with bandages covering his arms. The game and the multiple endings are hallucinations as Thomas bleeds out from his suicide that hint at the true story.

[XTYRMIN8Z]

when i read that title i was certain this post was going to be rediculously stupid. you proved me very wrong.

[Killafin]

incest is but one theory. I would also like to note since they would have a child in this theory it would actually be considered incestral (see what i did there xD)

[GLaDOS]

Magnificent backstories!

[Android]

if he is adopted I dont really think it counts as incest, since they arent blood related.

[GLaDOS]

true

[Theredshirt]

I just thought that all three of the ending were cannon, I thought that they both grew up together got marred and had a daughter, however their daughter died and their marrage fell apart, Thomas in his depression tries to kill himself which causes this game to happen with all these nightmares until he awoke to find his equily depressed ex wife begging him to wake up.

[Emol Dandriel]

Nice theory.

[Hypno_Wolf]

This sounds really interesting, good job.