His new girlfriend is not okay us to be friends and forces him to not talk to me, but why does he do it in secret?

Anonymous
We broke up 3 years ago, very peacefully and remained friends. There were some values that he did not agree, he was more traditional.
He did not want to ever lose me so it was his request to break up + stay close friends.
We remained good friends for the next 2 years, he was even more open and initiative after we broke up, even more flirty. Then he got in a relationship and his new girlfriend did not like our friendship (even though there is nothing between us), and wanted us to stop communicating. He was sad because of her not trusting him and they have turbulent relationship even though me and my ex really stopped communicating, wanted to keep out of their relationship because I did not think I should have been involved.
My ex still texts me in secret, and tells me he is not doing any harm by talking to me. He tells me he is unhappy because of his partner who is distrustful and stresses him out, distances him from other connections. He tells me I understand him well.
Any options why would he be so protective of our relationship? Sometimes I feel like he's attached to me even though we did not communicate for 6 months, they still keep fighting about me. And my ex does not want to end with me even though I'm distancing myself and just trying to support them but I feel it's their business.
Why would this guy be doing it? do you think he has feelings for me?
We broke up over a silly reason - he said he was not ready for sex outside of marriage and since we were having it, he did not trust me to be 'pure' in my past. He told me he felt better off being friends and more trusting in this. He is not having sex with his current girlfriend, she's traditional as well. Do you think he might be having sexual attachments towards me?
His new girlfriend is not okay us to be friends and forces him to not talk to me, but why does he do it in secret?
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