I neutered my male at 6.5. He was having prostate issues. It helped but he still has flare ups of urination problems (read standing and dribbling for 10 minutes). The vet did a full work up, x-rays etc. The next option was neurological. It came down to trying one or a combination of 5 different drugs. I didn't think the symptoms warranted the "shotgun" approach of treatment.
So, unless you have intolerable behaviour (humping etc.) or a health issue I'd let him keep the twins. As far as unwanted pregnancies, a fenced yard or kennel works wonders.
Brian