For clarity amongst our non-gliding brothers, a very high percentage of privately owned gliders are single seaters. A very high percentage of the two seaters are owned by the gliding clubs.
The reasons tend to be
a) price of a 2 seater is higher than the equivalent performance single seater
b) insurance costs are close to double for 2 seaters because of the 2nd seat liability
b) rigging and derigging 2 seaters is much heavier than singles - club-owned aircraft are generally kept rigged in the hangar, private gliders are generally stored derigged in their trailers
So if you want to fly wife/girlfriend etc etc it usually implies using a club-owned glider and following club rules on who can or can't.
I agree entirely that doing a BGA basic instructor course does an awful lot of good for your flying and a Half Cat rating even more.
Our club uses the "instructors only" rule for club 2 seaters. There are a few vociferous non-instructors who consider this unfair but at least one of them I wouldn't trust with flying my mother in law.
If a non-instructor wanted to bring a 2 seater onto our site, he would need permission from the CFI. This would not be granted except to pilots of known ability.