Grant Larsen – who cheated on his girlfriend Ashley on the Netflix reality TV show “Temptation Island” – is opening up about his main reasons for questioning their relationship on the show.

Grant tells TMZ … he worried Ashley didn’t have the patience and kindness to be a stepmother to his child, and while he spent hours talking about this with other contestants and in his confessionals, those conversations didn’t make the final edit.

Grant says Ashley didn’t treat him with respect and wasn’t “soft” with him … and those were red flags, which had him over-thinking their long-term relationship, leading to his infidelity.

Another red flag … Grant tells us the first time they met in Punta Cana, in the Dominican Republic, Ashley made him approach other women to see if he could get them to be interested in him — and finds it ironic that 18 months later, she asked him to go on a show that pretty much would make him do the exact same thing.

Despite these concerns, TMZ has learned Grant and Ashley got back together for 6 months after the show, and he bought her an engagement ring … but never proposed.

We obtained videos of the engagement ring — check it out, looks pretty big!!!

Grant and Ashley ultimately split, and he’s been spending his time working on creating a daddy-daughter swim line called Amora Beachwear … his brand partners with BetterHelp and Blessings in a Backpack pledged to donate 10% of every purchase to charity.