Even though I think I turned off the new tumblr dash, something about its interaction with Xkit has changed such that I see recommended posts now. I've adapted to them fairly well, but there have been several now in the genre of asks that are like "how can you have mixed-orientation relationships?" and I'm just. Tired.
As much fun as theory is, the truth remains that relationships happen between people, not ideals or archetypes or character stat sheets.
Every interpersonal relationship is a series of questions and cost-benefit analyses and choices. A decision to categorically include or exclude certain people from your dating pool will last precisely until you meet an individual who you want to date. That sort of thinking does not serve you or the people around you well, not least because it's only helpful at that very first stage of "should I form a relationship with this person?" and not at any point thereafter.
What you should do instead is learn about yourself, your goals, your boundaries, your needs, your strengths and weaknesses, what would make you happy. Develop your ability to communicate all of these things to other people and to hear and process the same from them. Then when the questions and cost-benefit analyses and choices come up, what you have is a framework for making good decisions. You've given yourself a set of tools instead of opaque, inflexible, authoritarian rules. Which, P.S., will work for all relationships, mixed-orientation or not, for problems to do with orientation or not.
Now, I am not proposing this will make you perfect or infallible in some way; sometimes the answer is to not embark on the relationship, or to change its nature, or to go your separate ways. But what it should do is to allow you to make those decisions with confidence and with minimum regret.
I am also not asserting that it is easy. Certainly I have been blinded by my attachment to what I hoped or wanted the outcome to be in the past and stayed in a relationship too long. It is possible to do nothing wrong and still lose. But again, that question of "what if things change in the future?" is not productive. Things will change or they won't. All you can do develop your tools and make the best decision with the information you have at the time.