Why are gay people not allowed to get married if a couple who cannot have children can get married in Church?
Because the Church holds that the marital act is coitus, not other sexually erotic activities. Impotency, not fertility, is an impediment to marriage.
Also after a married couple over 50 can continue having sex, even without the ability to have children.
It’s rarely talked about, but if a couple becomes permanently unable to engage in coitus, it is expected that sexual activity in their marriage stop. I’ve heard an elderly woman suffering very a vaginal prolapse that didn’t get fixed adequately through surgery being told by a priest spiritual director on Relevant Radio tell her that what she and her husband were doing was not rightfully called sex. They hung up on her expressing she was getting too detailed and that children might be listening.
Is it because allowing a gay couple to get married in the church would challenge the use of condoms (I understand the pill is different in this case).
Well, if a couple engages in other sexually climatic activity, it’s considered a sin. This is whether they’re indulging in it during their fertile window to avoid pregnancy or in other occasions unless circumstances force them to render the act incomplete.
To be honest, I’ve struggled with this one as well since I’ve been married. I don’t find coitus to be the pinnacle of sexual activity, the thing that all sexual arousal leads to.
That said, we are all called to chastity according to our state in life. We have very heteronormative ways of promoting “chastity” which are really rooted in avoiding the opportunities to sin, not in actually cultivating self control. We also obsess over the sins of lust. The way I see it, traditionally, lust and gluttony have been considered the least of the seven deadly sins. Chastity is like temperance with food. You should strive to eat a healthy diet just as you should strive to live a chaste life.
Eve Tushnet has some great insights. The video is long though