"I want you all to know why I went on this TV show and why I went back for @alabamahannah after she sent me home," Luke began.
AsThe Bachelorette viewers recall, Hannah eliminated Luke before the overnight portion of their Fantasy Suite date in Greece because she felt judged and shamed by the religious bachelor during their discussion about premarital sex in which Luke questioned whether Hannah had slept with one or more of his competitors.
But Luke returned to the Final 3 Rose Ceremony to fight for Hannah and hopefully win her back -- only to get brutally rebuffed by Hannah again in front of Tyler Cameron, Jed Wyatt and Peter Weber.
"I never wanted to go on the show to begin with, my sister in law @hleaparker22 signed me up and I kept an open mind," Luke continued in his Instagram post.
"Going through the interview process I watched [Colton Underwood]'s season and a beautiful southern girl stood out to me during the intro videos."
"She told the cameras she respected Colton's virginity and that it was super appealing to her that he was a virgin, and was the bachelor because sex was a big deal to her," Luke explained.
"I saw her Instagram and in her bio read 'I love Jesus with my whole heart', I was sold and if I was going on this TV show it was for her."
Luke therefore appeared on the show as a born-again virgin. Despite abusing sex and alcohol in his teen and college years, Luke had abstained from sex for three or four years prior to going on The Bachelorette.
"The first night I made her a Proverbs 31 bracelet then explained to her that I noticed what she said to Colton and I knew where it came from and how important my faith was to me. Shortly after that, I get the first impression rose," Luke wrote in his post on Monday night.
When Luke and Hannah's romance crashed before their Fantasy Suite, he was shown saying the marriage bed should be kept "pure."
"I just want to make sure you're not going to be sexually intimate in your other relationships here. I totally have all the trust in the world for you, but at the same time, I want to know we're on the same page," Luke told Hannah at the time.
"If you told me you're having sex or you had sex with one or multiple of these guys, I would be wanting to go home -- 100 percent."
But instead of being on the same page, Hannah flipped out on Luke and told him to leave. Hannah admitted she had sex in a windmill -- twice -- and even flipped Luke her middle finger on his way out.
Luke explained in his Instagram post there was a reason why he felt so convinced Hannah was not having sex in her Fantasy Suites when he broached the personal topic over their dinner date in Greece -- and why The Men Tell All also showed him claiming Hannah had said she wouldn't have sex on the dates.
"In my hometown date I brought Hannah to a bible study with my college church group, where I shared my story of how God transformed my life and gave me a desire to live my life for him in obedience, that included waiting until marriage for sex," Luke shared in his post.
"Hannah on the date, stands up and tells the group her story of how she was convicted to live a life in pursuit of Jesus. She also told them that she wasn't going to use the fantasy suites for sex but for conversation, no matter how @abcnetwork made it look."
During Monday night's The Men Tell All, Hannah insisted she had never planned on having sex in the Fantasy Suite -- that it just happened when it felt right -- but it was not Luke's business to ask her about it or anyone else's business to know.
Luke was then shown claiming Hannah had said she wouldn't be having sex during the overnight dates, which Hannah immediately denied.
"Are you really telling me that you never said, 'Don't judge me about the Fantasy Suites, because I won't be using them for sex?'" Luke asked Hannah.
"I did not say that. Fantasy Suites aren't used for sex. You didn't have one so maybe you don't know, but Fantasy Suites aren't used for sex," Hannah replied.
In his Instagram posting, Luke also attempted to justify why he returned to the Final 3 Rose Ceremony after being eliminated.
"I had to go back and fight for her," he wrote. "I believed in my heart she was making the biggest mistake of her life based on what she was telling me, I wanted her to know that we could get through this and I was here for her. #thebachelorette."
Luke's brother, Mike Parker, also recently defended Luke in an interview with Fox News host Todd Starnes during an appearance on his radio show.
According to Mike, Luke only believed Hannah wouldn't be having sex during any of her overnight Fantasy Suite dates becauseThe Bachelorette star had publicly stated that during a portion of Luke's hometown date beforehand.
"On Luke's hometown date when he brought her to a Bible study group with his college ministry... he had a conversation in front of the Bible study group. He shared his testimony [and then] Hannah B. stood up and shared a little bit of her testimony and shared that she wasn't going to be using the Fantasy Suites for sex, that she was going to be using them for conversation time to further her relationships with the guys," Mike told the Fox News host.
"This led Luke to believe that they were on the same page about sex," Mike insisted.
"So when you see this all go down [on the July 15] episode, Luke believes that they are in agreement. You can see the shock on his face. You can see the portrayal that they're trying to put Luke in -- that he's shaming her, that he's slut-shaming her."
The Bachelorette's Season 15 finale is scheduled to air as a two-night event next week, with Tyler, Jed and Peter still in the running for her heart.