Wh
y do I call the union between Yuuri and Wolfram Yuuram and not Wolfyuu? The reason is very simple: It's easier to pronounce, and it's a cute name/term that Greta their daughter made up herself. Other than that, I wouldn't differentiate between them as to who is the uke and who is the seme. Their union, in my mind, should be like a yin and yang, each has a bit of the other, and each gradually turns into the other. Wolfram and Yuuri both possess interesting combinations of traits typical of both genders, and instead of pinning them down to be either uke or seme, I'd rather think that they alternate the roles between themselves. Each has his own moment of being the yin or the yang, each has his turn to give and to receive. If nothing else, their life would be much more interesting that way.
Wolfram is described in the novel to be stronger, manlier, and much more mature than Wolfram in the anime. He's hot-tempered, but he's far from being the loud chap who gets possessive over his love object and throws tantrums anywhere anytime as depicted in the anime. In the novels, he comes across as an experienced soldier and a patient and understanding friend (which can only come from being mature). In fact, his 82 years of life experiences hadn't all passed in peace. He had gone through a horrible war, during which he was personally involved in battles (instead of being locked away in the castle as shown in the anime, if I remember correctly). He had gone through the ordeal of having his own family member (Conrad) sent to the front in an extremely risky situation, as well as losing the person who was close to him (Julia, who's supposed to be his tutor on healing magic). In other words, he has seen a lot of ups and downs in life, and that's bound to make him a mature person. It's difficult to believe that someone with 82 years of life experience is not more mature than a 15-year-old boy. And in my eyes, some of his interactions with Yuuri clearly show him to be the stronger person, the pillar supporting the other boy.
However, utterly contrasting with his fierce personality, he chose a frilly pink gown to be his night dress. Some hypothesize that he only does that for Yuuri, to stress on his feminine look, since he knows Yuuri likes girls. But I personally wouldn't think so. I would just think of it as one of his idiosyncrasies, a cute and lovable one ;-), one that definitely shows a bit of his feminine side. The other traits that would make me relate to his feminine side is his caring, devoted and passionate nature, often stereotyped as the strong points of a woman. I have yet to see him being emotional in the novel (as opposed to crying when Yuuri had to leave without coming back, or sitting by Yuuri's bedside, holding his hand worriedly while he was unconscious), but for sure he gives in to his emotions much more than say, Gwendal, and that is again often stereotyped as being a woman's trait.
On the other hand, Yuuri is the simple, friendly, soft-hearted guy who always needs protection and who gets scared much easier than Wolfram. In their normal day-to-day interactions, he might be categorized as the weaker side, since he frequently got yelled at and bullied by Wolfram for being wimpy. However, when he needs to, Yuuri is totally capable of putting his foot down and bringing Wolfram back in line (one such situations comes to mind: during their adventure in Holy Sand land, when Wolfram attempted to trick Yuuri so that he would rendered Conrad and himself over to Yelsi instead of Yuuri). He's strong and manly in his own way, as only such a person would have the heart to be so forgiving, generous and trusting as he is.
So, the conclusion is, I would not make Wolfram the uke just because of his pretty face, or Yuuri because of his soft-heartedness. I'd rather think of the relationship between them as flexible and ever-changing (in a good way), like "a moving sea between the shores of [their] souls" (KG - The prophet).