Have any popes explicitly expressed disapproval of the inclusion of homosexual priests, including the nonpracticing, into the priesthood? A 2002 Los Angeles Times poll of 1,854 priests in the US  found that 9% of the priests surveyed marked themselves as homosexual with 6% identifying as 'somewhere in between but more on the homosexual side'. These numbers are significantly higher than the general population's number of self-identifying homosexuals, which is around 3.8%. It is thought that a large number of homosexual priests entered the clergy in the 70's and 80's. 

Have any popes, with or without the knowledge of these statistics, ever clearly and straightforwardly frowned upon the idea of allowing homosexuals, specifically including nonpracticing ones, into seminaries? If so, what was the general thought process involved in this view?
There is a document issued by the Congregation for Catholic Education (which oversees among other thing the seminary formation of priests) issued in 2005 that goes into detail about policies for admitting those with homosexual tendencies to the priesthood: it has the rather long title Instruction Concerning the Criteria for the Discernment of Vocations with regard to Persons with Homosexual Tendencies in view of their Admission to the Seminary and to Holy Orders

This document is normative (i.e., it has force of law) for seminaries, and it says, in a nutshell, that those who currently have deep-seated homosexual tendencies should not be admitted to the diaconate. (Evidently, much less should they be admitted to the priesthood, since the diaconate is the first degree of Holy Orders, necessary before becoming a priest.) They should not be admitted to Holy Orders, because being a priest requires that a person have the ability to form healthy relationships with the men and women they encounter in their ministry. Deep-seated homosexual tendencies, which are not, of course, sinful in and of themselves, nevertheless place a serious obstacle to the kinds of relationships that priests need to develop with the faithful they minister to.

However, the document says that those who have clearly overcome such tendencies for at least three years could be admitted to the diaconate, provided the other elements that go into vocational discernment are in place: desire on the part of the candidate to be a priest, a positive judgment of worthiness on the part of his formators, a healthy moral and spiritual life, a good prayer life, and so on. (In most dioceses, transitional deacons are ordained priests a year or two after their diaconal ordination, the minimum interval being six months.)

The document is very thorough and succint, and it serves as a good explanation of the Catholic Church’s teaching on homosexuality in general, as well as how the Church applies that teaching to admitting men to the priesthood.

As regards the statistics, I am sure that the Congregation for Catholic Education is well aware of them, and it is doubtless partly in response to them that it issued this document.

(I should point out that the L.A. Times poll does not appear to be scientific, and it seems to me that the numbers are rather high. Nevertheless, the Church does recognize that there is a historic problem of admitting men with homosexual tendencies to the priesthood, and has moved to stop it, as can be seen.)