CCC 1654 Spouses to whom God has not granted children can nevertheless have a conjugal life full of meaning, in both human and Christian terms. Their marriage can radiate a fruitfulness of charity, of hospitatlity, and of sacrifice.
Marriage is centered around procreation and upbringing of children, but infertility does not diminish marriage unless it is chosen infertility. Homosexuals may not be able to choose their sexual orientation, but they can choose whether or not to have sex. Homosexual sex is just as much a chosen infertility as is artificial birth control and sterilization.
There are modern day cases in which infertile heterosexual couples conceive. I have not once heard of homosexual couples conceiving through sexual acts.
ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=7789569&dopt=Abstract
Because it is possible for infertile heterosexual couples to conceive, an ‘infertile’ couple wishing to marry can still have children, thus they are still open to life. Perhaps one exception to this would be if the couple would not want to get married if they were fertile and could produce children, in which case they would not be open to life.
Yes, God could cause lesbian couples to conceive, but there is no evidence of this ever happening. Until there is evidence to support this, I have to cause to believe he will ever do so.
Peace