In our church, we become pretty familiar with the word stewardship. It means you have a right to receive revelation for whatever it is you have stewardship over. In my opinion, which is based off of what I have read from conference talks, church publications, etc. is that husband and wife, together, have stewardship over their sex life - and no one else, really. Others can give advice, but God gives revelation, where needed and asked for, to the husband and the wife - not to others.
In thinking about what's okay and what's not for married sex, my rule of thumb is: revelation. There's a quote that floats around almost as much as the 25-year-old menace to society, that is, "if a person is engaged in a practice which troubles him enough to ask about it, he should discontinue it." I don't think that's a great thing to live by for a lot of people. Personally, I ask a lot of questions and do a lot of research because it is in my nature to want to know things. Consider this: someone of another faith is thinking about joining the Church of Jesus Christ of Latter-day Saints. They act on this line of though by attending the church and taking the missionary discussions. They decide to ask about whether or not this is the right thing to do. They ask Heavenly Father. Does the fact that they want to ask mean they should discontinue those things? NO! Clearly, this is not a universal truth, and I think a better way to say it might be something along the lines of, "if it makes you feel guilty and want to repent, you should probably not do it." Because that's probably revelation (unless you have a personality that feels guilt for no reason, which can sometimes be the case).
So, for example, when my husband and I were engaged, the topic of oral sex came up. I had wondered about whether it was okay, and the quotation I showed above struck me hard as "maybe that means I shouldn't even think about this." Because I was curious and wanted to know more about it and what other LDS people though - okay or not? I did like the idea of it and wanted to try it. I didn't feel particularly bad about it, only worried that that quotation would condemn me. In the end, we decided that since we both felt comfortable with the idea, we should pray about it, and see what Heavenly Father thought. After praying, I felt a lot better, and the idea came to me that we could try it once, and if either of us felt guilty or dirty afterwards, we should not do it again. But if we both felt good and loved and happy, it would be fine. Guess what? It was the second thing in our case.
On the other hand, we will probably never try anal sex, because I am uncomfortable with the idea. I don't need to pray about it, I know I don't want to do it. And if either husband or wife feels uncomfortable with a certain practice, it might be best to not do it. This is, of course, not a universal truth either - but I believe it applies in many circumstances regarding sex. As long as it isn't taken to the extreme, like "sex makes me uncomfortable, so let's never have it." That probably means you need therapy.