I have never had any problem with mature adult serious couples on the mat. I think 19 year old couples (like me) are terrible and not mature enough to handle situations like break ups and fights
I was 18 when i started training with my bf, 19 when i passed my 5th kyu, which was 15 days before Matthew proposed. Never judge by age alone, we get on well on the mats. i'd be heart broken if i was told i couldn't train with him. I mainly train with others, but do a technique or two with him aswell. He helped loads getting me ready to grade for 5th kyu, he got his the day before me. I think couples who aren't allowed to train together at all are being deprived. Obviously to train well you have to train with many people of many levels, but being able to train with a partner brings in a different element to training. If you're angry with your partner avoid traing with them until you're calmer, but i don't feel people should be banned from training together encase they behave foolishly. But i believe in innocent until proven guilty, not guilty until proven innocent. If there is a problem couple, then split them sure, but don't do it unnecessarily.
I know a lot of 19 year old who are mature enough to handle aikido with a partner, and a lot of older people who aren't.