I have a halachic question of a sexual nature that I feel I can't ask my rabbi. Where can I get such questions answered? Are there rabbis knowledgeable in this area who are willing to answer questions from strangers?
I feel a bit of sadness in writing this, because I think it's something that should be - but isn't at all - obvious.
You can certainly seek out rabbis who can give general answers about Halachah without knowing you, perhaps without ever meeting you, but you shouldn't. You should, instead, seek out a rabbi with whom you can be comfortable asking these questions. Maybe in your case it should be a rabbi you don't see every day. But it should be a rabbi you know - or get to know - well. The best answers, especially in such sensitive areas, come out when a rabbi knows just what is going on in the person's life.
If you can be comfortable speaking with a therapist about these issues, you should feel equally comfortable (perhaps more comfortable) speaking with your rabbi about them. Your rabbi should understand you, and you should understand that your rabbi cares about you and will help you in a very professional and mature way.
But this isn't how most people view rabbis' role, which is why I said I feel sad writing this. It should be so obvious that it needn't have been written (and so obvious that the question needn't have been asked). But, sadly, to most people it's not.
I can't quote him verbatim, but Rabbi Ahron Lopiansky once said something along the lines of 'the only thing keeping people from getting advice in Torah from rabbis is the people themselves'.
Sexuality is a part of Torah. Rabbi Kahana went as far as to hide under his teacher's bed to learn proper conduct with one's wife. Rabbis know this. They aren't going to gossip about you or judge you, and they have no reason to. If you are seeking guidance from Torah, in any area of life, there is absolutely nothing to be ashamed of. If anything you should be proud of yourself for you sincere commitment to Torah, even in private areas that many choose to remain ignorant of because they can't get over their embarrassment, or they don't want to hear the word 'no'.
If you have rabbi that is 'yours', as you say, I highly recommend asking him your question. He knows your personality and struggles and will be best equipped to teach you what you need to know in a way that you can understand and readily accept. If you really can't bring yourself to ask him, any other rabbi would be happy to answer your question, (or refer you to a rabbi who can answer your question), discreetly and politely. I wouldn't ask in line after minyan, but most shul rabbis have hours where they can answer questions, sometimes even over the phone if you are still too uncomfortable with a face-to-face.
Certain questions take some gumption. You have to contemplate and internalize the fact that learning the proper path for a Jewish life is more important than a few moments of contrived embarrassment with a rabbi.
I wish you well and hope that you are able to comfortably find answers to your questions!
I agree that one should ultimately find a rabbi to whom he or she is comfortable asking any questions they might have. If you don't yet have a rabbi that you feel comfortable asking these type of questions try sending it in to my site, pocketrabbi.com, a system built for answering any halachic shailas. We have a staff of great rabbis that will do their best to try and give you your answer and maybe you can get to know one of them and they get to know you.