Well, what's the alternative, really. A few things could happen:
Couple have a first date, decide not to go forward.
Couple goes out on first date, are on the fence, get genetic testing -- testing says they're incompatible, they break up.
Couple goes out and are really really into each other. Then they start to think about genetic testing.
A few things can happen here:
- They just woops, keep pushing off genetic testing, we'll do it one of these days ... maybe ... but hey shouldn't we just have faith in G-d as clearly we belong together ... (witness how difficult it has been to get everyone onboard with genetic testing).
- They get the test and break up, causing a great deal of heartbreak and emotional scarring.
Not good. Would have been best they hadn't gone through all this dating.
- They're awfully, awfully torn; finally their genetic counselor says "well the best thing is to just get married and then do amniocentesis, you can always terminate the pregnancy if it turns out to be Tay Sachs."
The couple has now put themselves into a situation where keeping halacha will be of nearly superhuman difficulty. Not good. (This is the reason R' Moshe Feinstein and R' Moshe
Dovid Tendler aren't crazy about testing fetuses -- there's nothing you can do about the results that's within the bounds of halacha, according to them, so why torment yourself?)
So given the probabilistic expectation here, it's very, very wise (and therefore recommended) to get tested first. Are we absolutely black-on-white required to avoid unnecessary situations that cause heartbreak? Or that make keeping halacha incredibly difficult? "Very much should" is good enough for me.
It's about what makes for good policy, really.