Back in my good old not religious days, I absolutely touched and was touched by men. That's not to imply I would mess around with any old guy just for the heck of it - this was in the context of (at the time) an emotionally close, intimate relationship. Those relationships are over now, but I'm glad I had them as they were for so many reasons. Some of the most important lessons concern what I learned about myself, what I learned about dating, what I learned about friendship, and what I learned about emotional vulnerability and connection. And, some of the most important lessons I learned were lessons about how touch and emotion and friendship can interact. While none of these relationships turned into marriage, all for a variety of good reasons, I'm lucky to say that I have experienced just how wonderful all of those things can be when they interact in just the right way. Touch isn't something ancillary to an intimate relationship, it is part of the relationship.
An important aspect of dating, even when dating for marriage, is a learning process. You learn what you need in a partner, want in a partner, and refuse to tolerate in a partner. You learn how to be a partner, how to be empathetic (in a way that simple friendship doesn't include), what you need to give, what you can try to give, and what you can't give. You learn to forgive. In addition to these components, if you touch each other, you learn how to touch. You learn what is affectionate, what is passionate; what is appropriate, what is not.
(For another post, perhaps: my sense is that shidduch dating tends to inhibit a lot of this learning process, particularly on the emotional side. By having a gigantic questionnaire / screening process / chaperones, it makes it difficult for the potential couple to really see each other and have their quirks bump up against each other.)
Shemirat negia cuts off a whole avenue of learning in dating, and it means that by definition, a couple has built a substantively incomplete relationship prior to getting married. Some people (like yours truly) are very protective of personal space. Beyond family and socially mandated touching (like shaking hands at a job interview) the idea of touching someone, anyone, for any reason is a whole hurdle for me to get over. It's hard, very hard, for me to imagine sex with someone as a pleasant act, at least and especially the first time. As part of partnership, I want to have a good sex life with my husband, if and when I marry. Unfortunately, one result of keeping negia is that a couple can't work on this aspect of the relationship until marriage.
I'm not trying to pass judgement on whether shemirat negia is or is not halakha. Even the most lenient of readings seems to forbid sexual contact between unmarried partners. Still, it seems that it has a number of detrimental side effects.