For some same sex attracted priests, communal living may be too close of an occasion of sin.
It was certainly proven to be too close of an occasion of sin when this happened with great frequency in the '80’s. (i.e., significant critical mass of SSA seminarians) The result was not just internal scandal, but – perhaps more importantly (and referred to earlier) – critical morale problems/discouragement on the part of heterosexual seminarians, due to the acted-out inclinations of their brothers.
There are some dynamics that are important to remember here:
(1) any group that has been, or has perceived itself to be, an ‘injured’ or ‘oppressed’ minority, tends to react in larger group settings by adhering to each other more intensely, more cliquishly. This was proven to be a dynamic, again, in the '80’s, where there were gay “subcultures” within the seminary resulting in a sense of exclusion for straight seminarians. Not only is this unhealthy for seminary formation,
it is opposed to Christian charity, in that community is an essential feature of religious life – whether vowed or diocesan. So there were both theological/spiritual and practical effects from admission to the seminary of those who identified as SSA (and often acted on that!)
The further theological, spritual, and practical ramifications of this were addressed well by JReducation in a much earlier thread, wherein he discussed how and why formation directors weed out those men who have such a “prejudice” (favoritism) for their own gender that it limits their ability/inclination/desire to relate to the whole mixed-gender People of God (parishes, academies, more). There are gays and there are gays. We’ve all known gays who are affectionate toward the opposite sex and enjoy the opposite sex immensely. Unfortunately, that (again) was not the pattern in the '80’s. The gays attracted to seminary life did not include (in my personal acquaintaince with that history) very many who even tolerated the opposite sex much, let alone viewed them as equals, as having separate dignity, as having equally legitimate needs, as worthy of affection, etc. Rather, for these gays, seminary became an Opportunity Club for them: at the very least, an opportunity to be exclusive with other gays; at worst, an opportunity to have sex with selected others, and even some cases

with a group
(2) Most men view sexual desire differently than most women do. (I stress the word “most” in both cases.) There is a “determinism” about culmination/consummation for most men that heightens the difficulty for them in postponing that indefinitely or permanently, when attraction develops. That appears to be inborn and is likely a by-product of the biological drive to mate. It’s not that it’s impossible; of course not; otherwise, there wouldn’t be both SSA and non-SSA celibates among the clergy who have been able to postpone that, sublimate it, etc. However, again, it’s a matter of dynamics when a critical mass of candidates who openly identify as SSA, and who discuss their sexuality and their “gay identity,” etc., enters an institution.
When, by not addressing the issue, tacit “permission” is given to one group of seminarians to “be who they are,” then by extension the logical result is serious temptation to ‘culminate’ relationships. As another poster said, this is why celibate heterosexual priests do not share a rectory with unattached heterosexual women.
I do disagree with the poster who implied that gay men are more ‘sexed up’ than straight men. I think the two groups are equally sexual. (There is a small subset of both males and females that are more ‘asexual’ than the rest of us and have a high tolerance for ascetism in general, but they are a tiny percentage of the population.) The difference is, straight seminarians are discouraged from sitting around talking about babes they would enjoy mating with. In the '80’s, there was a double standard operating in many seminaries. So I think the current prohibitions are designed partly to get separation from that recent history.
(3) I think the contemporary world, as has often been noted on CAF, and formally discussed by Benedict XIV, is sexualized to an extreme. That means that many of us have to go to extreme measures to avoid near occasions of sin, which abound without our looking for them. That, combined with the modern tendency in ‘identity’ movements to define sexuality as identity, multiplies the difficulties for admitting openly gay seminarians, particularly those who have practiced actively in the recent past or for any prolonged period of time – such as living with a lover, etc.
(edited to say that I meant to include both the '80’s and '90’s)