“a shortage of sperm donors”
This means that there are insufficient sperm donors, not that sperm donors are lacking in height. Masturbation does not stunt your growth. I am six feet tall, and although I have been referred to on many occasions as “the biggest wanker in North East Hampshire”, it should not be construed that fellow practitioners are all midgets.
“The clinics are struggling”
This is all to do with meeting quotas, it does not mean that potential donors will be engaged in physical combat during the act of donation. That would be silly.
"It started to decrease even while the law was under discussion."
Then talk about something slightly more erotic.
“too few men were coming forward”
This is indicative of a lack of volunteers, not some bizarre onanistic position.
"Today, many NHS clinics are finding it hard”
Needs no explanation.
“a lot of sperm banks have closed their doors.”
This is probably a positive step. It may discourage a few exhibitionists, but generally these activities are best kept private.