Not knowing your particular situation any more, I cannot give you direct advice. However, in situations like yours, the question to ask before what should you do is about what are you going to do something.
It is clear that he is not returning your phone calls and this is something that hurts when one partner in a relationship has unilaterally pulled away. Are you wanting to have communication because you still want companionship even at that level? Is there something in particular that you want to communicate?
In situations where things have gone from great to bad, there are probably things that could have been caught on the way. These are lessons others can learn from your situation, but are not things for you to beat yourself up about right now. If there is something that you can do to rectify things that have happened, these are things you might chose to focus on now, if you are wanting to restore the relationship. If the things are now water under the bridge for both of you, then allow them to be water under the bridge.
When one parent in a separated situation has the children, it is not uncommon for the children to either become a pawn or casualty in the situation. If he is not wanting to face talking to you, he might not try to ask about the kids or even try to call them. Assuming both of your children are younger, being careful to be appropriate about what you tell them is important. What is generally not a good idea is to lie to the children, even if well intentioned. The parent who has the children, if they feel contact between the children and the other parent is important (which it generally is), may have to make extra effort to ensure that the contact is made. Maybe, if the child is old enough, help the child make a call to the other parent without you being on the call at all. Maybe, if the children are inquiring enough, have the children write a letter. Maybe later, if visits are arranged, you might have to meet at a neutral spot for the other partner to pick up and drop off the children.
Then there is the issue in your title - you say you are married to a bisexual guy. Your actual question does not refer to this at all. The questions you ask would be important whether your spouse was involved with a man or a woman. Either way could hurt and wither way could make you ask yourself why they are attracted to the other person and "what did I do wrong". These also are natural questions at this time.
Finally, how is this affecting you spiritually? What is your view of marriage and the fact that yours is now in trouble? How far do you feel (spiritually, religiously, emotionally) you need to go in trying to make the marriage come back together and work?
These are some thoughts that may prompt you on the way. Use your social network to help support you. If you need help beyond that, reach out for a professional like a marriage and family therapist. They can help you get a clearer view on things and decide on a course of action that seems right now and that you can live with over the long haul.
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