Ah, yah, let me say that first, I am just a normal person, with lots of life experience, and I want to help you clear the cobwebs in your head that we all get when faced with shocking and unpleasant discoveries...
Its normal to feel a sense of delayed reaction, numbness, anger, dissapointment, denial and finally acceptance someday, if this sounds like the stages of grief it is. It really is. You are grieving over what you thought you had, and what you were led to beleive you had...and worse than greiving a death, its still alive and kicking. It seems like these feelings come and go and not in any particular order...so, I'm going to give you some friendly advice that I learned from my experience.
I've said this earlier and it really helps to think of it this way: Other people do exactly what they want to do, in exactly the way they want to do it.
What does that have to do with you? Well, you're the other person, you have every right, as a live in partner for 18 months, to be told the truth, to be treated with kindness and respect. To be, as his partner, his sexual choice. In order to have a full relationship, there must be honesty in order to build trust.
It sounds like he lied to you, and may have assumed you didn't know better, or wouldn't figure it out.
It sounds like his secretive behavior has a certain 'bad-boy' thrill for him, that he hopes you will suspect, because thats part of the fun...maybe getting caught.
It sounds like he is skirting the edge of being wayyy out in left feild and doesn't know it...if you care to realize that his behavior isn't about YOU, then you may choose to end the relationship and find a partner who does sexuality like you do, and you mesh in that way.
Or you could slog it out with him, going to counseling together, and getting it all out in the open and working through it...either one is possible and no one will be able to tell you what to do...this is a personal choice for you to make. But, don't prematurely forgive and forget when there is no acknowledgement of wrongdoing, or attempt to change on his part.
Its just not working for you, and thats okay, its part of your core beleifs about what is right and wrong, and its different than his.