In all the talk about same-sex marriage recently, especially after President Barack Obama's somewhat late but still historic announcement about his 'evolving' views on the matter as well as Denmark passing an equal marriage law in parliament this week, a certain group of people with a vested interest have been ignored.

While gays, lesbians, and homosexuals are all terms used and people referred to in the discussions, bisexuals seem to have been forgotten.

Bisexuals, of course, might be in either opposite-sex or same-sex relationships, and if it's the latter, they too might want to marry their same-sex partners just as much as some homosexuals do. Bisexuals, too, have fought for equal access to marriage (and other rights) and yet seem to be regularly erased from the picture.

Much of this stems, I think, from societal biphobia and from fear of the challenges that bisexuality presents.

For example, it is safer to think that people are always "either-or". They're either black or white, Christian or non-believers, gay or straight, and so on. Of course the truth is that few people fall solidly into easily understood categories. Bisexuality reminds people that sexuality is more fluid than that.

If you accept that others might feel attracted to people regardless of gender, then that means you, too might feel that way. And that's scary. You might worry that one day you'll find yourself in a relationship with someone of the 'wrong' gender, and you wonder what you would do or what your friends and relatives might say then. So it's easier and more comfortable to just pretend that this isn't valid and that bisexuals don't exist. Doing this also makes the idea of marriage more clear-cut.

Bisexuals are also viewed as a threat because people erroneously consider them greedy and promiscuous. There's the false idea that if you're bi, you want both men and women at the same time. You're never satisfied and if you're with a man, then you'll soon be looking for a woman, or vice versa.

This mistaken belief about bisexuals causes stress in regard to the idea of marriage for a couple of reasons. One is that some people wonder if bisexuals can have a monogamous marriage, so they leave bisexuals out of discussions about marriage since they view monogamy as integral to relationships. The obvious point here is that marriage is different for all couples and while some might choose monogamy, that's not for everyone, and that's true for all people regardless of their sexuality.

Also, some people claim that allowing equal marriage means that society is heading down a slippery slope where one day threesomes can get married (or people can marry their pets, or their grandparents, and so on). Threesomes are often equated with bisexuality, so this fear of or distaste for a wider definition of marriage (one that allows for a variety of set-ups as long as they harm no one) extends into a fear of and distaste for bisexuals themselves.

Besides challenging people's ideas about sexuality, gender, and love, another issue here is that bisexuals tend to be rather invisible. If they're in same-sex relationships, they're viewed as gay, and if they're in opposite-sex relationships, then they're seen as straight. They have to speak up and label themselves as bi or else adorn themselves with bisexual jewellery if they want to be understood as bisexual. But since it's tiring and impossible to always do this, instead bisexuals get absorbed into other groups (gay or straight), and thereby their own needs and rights are conveniently ignored.

For these reasons and more, bisexuals tend to be left out of the debates and discussions surrounding equal marriage. But as society is slowly but surely moving towards a situation where all people have the right to marry whomever we choose, we must not ignore or fear bisexuals. Rather, we should learn from them.

If people can be attracted to others as more than just a particular set of genitals - and that's clearly the case - then that's a strong argument for allowing everyone the freedom to marry. Bisexuality thus is essential to the marriage debate.

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