Firstly, you are not helpless. You have clearly started to try and rationalise what has been going on. It is almost impossible to work out what is going on in someone’s head at the best of times. Harder still, or simply impossible if they don’t know either. Now, it is entirely possible that he is hetero romantic and sexually bisexual. The two can coexist in the same person. He may have sexual attraction for sex with a man, but not the man in of itself. In other words, he has what is almost a sexual fetish for sexual contact with a man, while also having a strong sexual desire for a woman. The former being something he feels ashamed of, the latter being something he is naturally comfortable with. The use of trans porn could be felt by him to represent what is almost a half way house for him. That somehow the fact that the people he is interacting with are felt to be emotionally women, while also having a degree of masculine presentation.
 You are still having sex because you feel you still want to. I presume that you feel that he also still wants to. In many of the cases where one partner realises, and or, follows through on their homosexual needs or desire, they can become hyper sexual for a time. Much the same as a heterosexual partner who is having a heterosexual affair.
Many people live like this. They live in MORs, mixed orientation relationships. There are obviously compromises and agreements that have to be in place that allow each party to be who they are, while also understanding what else is important in the relationship. This of course doesn’t mean that you allow him to do what he wants, with who he wants, while you live like a ‘dog in the manger’ waiting for your turn to enjoy him, when it suits him. That isn’t a relationship, that is abuse. The use of Grinder is a bit of a worry. To have access to ‘hook ups’ shows at least an openness to possibly follow through on them. He may be using you as a form of beard. One that allows him to explore his ‘other side’ only when there is a safe and acceptable home base from which to operate. This could mean that he would not actually have the confidence to fully commit to a gay identity when the safety net of a plausible deniability is there to catch him.
I think you have already started to look at what you value in the relationship. Weighed up in your mind what you would feel is acceptable and what is not. If not, then do this. Tell him that you are confused and possibly in pain due to what is going on. Tell him that you should matter in all of this. Because, never ever lose sight of that. It is either acceptable, or not acceptable to you, simple as that. The best predictor of future behaviour is past behaviour. Sexual orientation may change over time. But leopards don’t change their spots. If someone for whatever reason is, hiding in, or using a relationship for anything other than mutual benefit, then something has to give. Most of the people who come here are not doing so because someone else is gay or trans. They come here because they have suffered lies, deceit, emotional and psychological abuse, loss, grief, pain, confusion, and often, all of these things. All of the things we didn’t expect from someone we loved.