While runningdude was off the mark in his claims, josh987654321 is also incorrect in saying that it doesn’t matter. The Vatican has released clear directives regarding homosexual persons discerning vocations.
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
So as you can see, homosexual tendencies and the lifestyle are treated differently from heterosexual activity in candidates for vocations, because there is a fundamental difference. Homosexuality is intrinsically disordered. A man dating a man is entertaining a lie, even if they are chaste; dating is something which is supposed to lead to marriage, and there can be no marriage between two men. A man dating a woman is taking a step toward marriage. Even if they are unchaste and commit mortal sins in their relationship, these desires and acts are not intrinsically disordered but taken out of the context of marriage, cannot be condoned.
Part of the Church’s concern is for the well-being of the whole community of brothers or priests. We have seen in the past that the homosexual lifestyle infected seminaries and religious houses. Homosexual activity went on, quietly or openly, and even heterosexual men felt pressure to join in, or felt ridiculed that they would not. The directives are aimed at screening out people who would re-introduce this kind of activity, a real problem of sexual harrassment.
Men in the priesthood and religious life live in close quarters with each other and there are common areas which afford little or no privacy. So people with homosexual tendencies must be especially good at containing themselves if they are to live this way. Seminaries are not co-ed, nor are the religious houses or most rectories; heterosexual men will not usually have the problem of compromising situations with members of the opposite sex.
So while any man with deep-seated problems with sexual continence may be a bad risk for a vocation, I hope you can see why homosexuality is different, and has merited special consideration by the Vatican. Heterosexuality is well-understood over thousands of years, but the nature of homosexuality as something that used to be hidden and secret, but is now wildly popular and gaining in acceptance, is still being studied and adapted to.