Random people on the internet can easily be ignored, and if the OP does end up marrying his/her partner, it doesn't really matter what people on the internet said about it. On the other hand, the fact that the OP's mother disapproves of the relationship has the potential to sour family relations for decades to come. What if the partner finds out that the mother tried to get the OP to break things off?
Parents should not be giving this kind of advice, unless the situation is a truly extreme one. At most, they should be pointing out downsides, like how it might be hard to stay in the relationship despite changing interests and lifestyles.
Aside from that, keep in mind that when it comes to their children's relationships, parents are the worst possible judges out there.
The story actually reminds me of a girl I used to know. She´d been in a relationship with the same guy since her early teens, which she eventually broke off - mainly because her mother, who regretted marrying early herself, kept insisting that she "experiment" and get some experience.
By the time the girl had gotten all the experience she could ever want, she realized she still wanted the guy she used to date. Of course, he had moved on by then. But hey, at least she'd gotten the experience her mother wanted her to have - and the STD to prove it.
As far as I know, she's still single.