Kant's second formulation (or the "ends in themselves" formulation) says:
use humanity, whether in your own person or in the person of any other, always at the same time as an end, never merely as a means.
I know that the focus is on "merely" as a means. This question and answer says using someone as a mere means is abusing their rationality by getting them to act in a way inconsistent with how they would act when fully informed i.e. that Kant has no problem with entering into mutually beneficial rational contracts. This allows the situations of playing tennis with your buddy, or buying food from your local grocer as being just fine.
To put in my own words, if you allow someone to exercise their autonomy in deciding for themselves which goals are being pursued in which ways (presuming that they have all the information needed to make this decision), you are treating/respecting someone as an end. Your tennis pal and the grocer you buy from both fit this.
So to put this another way, as long as they consent, treating them as a means is also treating them as ends.
However, I've recently read of a hypothetical (I think it's "famous" but don't know who it's attributed to) of a society where the norm is that in a male-female sexual relation, the male seeks sexual gratification for himself without concern for the female partner's sexual gratification. Since this is the norm, the female partners do consent, but it feels wrong to say that this isn't morally wrong in that the female partners are being treated as merely a means (to satisfy sexual desires).
This leads me to ask: is there a constructive way to conclude whether someone is being treated as "merely" a means? Specifically, I want to apply this to combat sports like (kick)boxing. The common argument I've seen supporting the morality of these sports is the consent one -- but if I decide that consent being the end-all-be-all to this issue is morality isn't enough, due to the hypothetical posed above, then I don't know how I'd defend the morality of (kick)boxing.
I've no idea (after throwing away the supreme power that consent has) on a framework to decide if fighters are treating each other merely as means.