It's hard to say without knowing more. As with all things human, there are two dimensions to consider: nature and nurture.

Nature: the human brain develops by default as female and for a boy is influenced by male hormone production during gestation. If a mother having a boy gets stressed during pregnancy some of the precursors meant to make testosterone get used to make stress hormones instead, and those parts of the male brain that are developing at those times can remain feminine in their development. If the stress abates the rest of the brain can continue being masculinised. So males can express varying aspects of femininity depending on when the prenatal stress occurred (and which brain region/s remained feminine). So your significant other might be 'wired' for femininity in some way or another.

The nurture side is more difficult to explore because so many factors are at play. Say a male has a domineering mother, for instance, and the Oedipus complex emerges into a submissive streak in him. As a boy grows it becomes harder to imagine himself as something submissive, because he's growing too strong to realistically fantasize about being subdued by a female, so he starts to fantasize about being a female to allow a continuation of his fantasy world.

Another possibility is that his parents reprimanded 'girly' behaviour too harshly and it became something really taboo. Then fear gets powerfully wrapped into doing anything feminine and sex gets amplified by the fear. Then the fear/sex partnership becomes something so intense it takes on an addictive quality.

As I said, it could be a lot of things. Have some fun with it. Explore it. It might be a fad. Or it might not. But you have an opportunity to explore something new and different. If you decide down the track it's not for you, you are free to voice your concerns and go from there. It may seem like a perversion, but all perversion is caused by something, and no one chooses it, so it's not really fair to judge people about it.