In my humble opinion, this impression is caused by selection bias. There are 4 possible scenarios for the development of a relationship.

1. They do not have sex and the relationship lasts a long time.
2. They do have sex and the relationship lasts a long time.
3. They do not have sex and the relationship lasts a short time.
4. They do have sex and the relationship lasts a short time.

From what I've seen, all these scenarios are perfectly possible, and to be frank, I think having sex or not does not impact the length of the relationship.

However, many people have the following perception:

If 1 or 2 happens, people don't think much of it. The couple is happy and all is fine.

If 3 happens, people ascribe the breakup to other reasons, maybe character differences or arguments or whatever. However when 4 happens, people tend to ascribe the breakup to having sex and "use and dump" etc. Now this is obviously selection bias, as there are many possible reasons for a breakup, even when sex is occurring.

Now, the thing is "use and dump" may only consist of 10 percent of the breakup reasons, but people keep focusing on them and talking about them. Other reasons like incompatibility, falling out of love or bad tempers tend to be regarded as "mundane" and not worth noticing.

The analogy is that shark attacks kill less than 100 people every year while asthma kills millions, but shark attacks get big headlines in the newspaper.