I've been thinking about this topic for some time. It's very touchy because there's not a lot of information out there and in anything vague like this there are those exclaiming either it is impossible for twin flames to be same sex and such such...I would like to say that first twin flame relationships are not solely based on sex. It is not sexual gratification because twin flame relationships are consider a powerful spiritual union that often happens to serve a higher purpose. People say it occurs between the masculine energies and feminine energies. It's the union of the dual forces so to them obviously it should only be between male and female pairs, but that is not the case. A female can have a prominent masculine aspect to her and a male can have prominent feminine aspects. When we are talking about spirits we aren't tied to the physical gender of the body. When we hear masculine and feminine, we tend to push our own stereotypes on them based on gender roles of society. It isn't fair to say that it is impossible that twin flame to be same sex. It comes from old ways of thinking that only hampers the collective forward effort happening at this time. Spirit has no gender as it is energy. It is entirely possible for a person to enter this physical plane as one gender or the other and their pair maybe the same gender. Perhaps they had lessons they needed to learn as one gender and not the other. I don't see why that would be problem other than the lesson they would have to learn and overcome because of the society they live in that might be against a pairing of same sex. How I see it when it comes to pairing, either one of the two may take on the masculine or feminine aspect. I believe they switch back and forth considering they are whole spirits and not just half.
Tangent ahead -->
Another thing people don't understand is that twin flame relationship isn't about fulfilling you and making you whole. Otherwise you'll just enter in another karmic relationship in order to learn what you need to be able to be the whole person. If you imagine we all carry both aspects to varying degrees within us. We reflect the dual things within this universe so it's not we need a twin flame to complete us rather we are coming together for a higher purpose such as raising love in the universe. It's a shame a lot of people think that everything will be better once their twin comes into their life. You first need to understand yourself and come to an understanding of love before you can come together with your twin. It will not be a romantic fairy tale and it won't be about just the two of you, but how the two of you are in relation to the universal scheme.
I also think that soul mate relationships are not always lovey dovey. They can be quite taxing because they make us grow. Some of us will spend our whole life here with a soulmate while others may just pass by so many until the lesson they are meant to learn are learned. Whichever emotional extreme these soul mate relationship take us, ultimately they are to make us grow until we can reach a place where we are one with the Source. Soul mate relationships don't even have to be romantic they could be platonic, sibling, parental, etc...