It is a 'self-evident fact' (we like those, don't we, dwyer?) that an awful lot of women like seeing 2 men together, with some in that category preferring more 'feminine' men (a lot of yaoi) or various types of traditionally 'masculine' men (be they bears (which is traditionally aimed at gay men in porn but lots of women like too - the equivalent japanese term would be bara but it's my understanding that's found offensive but some still use it), or your standard muscle stud guys) and everything in between.
The reasons why can be and have been pontificated over time and again. One suggestion is that many women like porn just as much as men (which is something men often deny) and that seeing it between 2 men removes the very uncomfortable realities of the way society/patriarchy/men set up power relationships between men and women; it becomes a free space in which women can enjoy sexual content free of the patriarchy that oppresses them and their sexual lives more specifically.
That's just one possible reading. It might be true for some women. There might be a smidge of that there for most women. I don't know. What is self-evident, however, is that the women who like reading and watching m/m porn and smut find it fucking hot.