I'd have trouble disagreeing more. The dating culture in Japan is different from that in America, but starting around university age pressure to date actually seems very intense. 合コン (Goukon) are a major driver of social interaction among university students and young workers.Further more and to my knowledge Japanese, especial young adults, don’t emphasise on dating like westerns do. They are keen to experience independence and craft their personalities before they even consider the idea of being with another person. Besides, most of us know the standards of a woman/girlfriend/wife in Japanese male dominated society.
Pressure on women to marry young isn't what it was in the 80s, but it's still a world apart from (for example) the USA where women who aren't even considered to have "put off" marriage still wait until their mid 30s. Before those 20 year old women marry they DO date, and they DO have physical contact with men. Idols are removed from normal society by those rules/expectations, and in general will feel like they are giving something up for their career.
That said, I do agree with the basic premise that they're not disbanding primarily over a desire to date.
I have some private theories about some of the bigger reasons for the breakup but the one I'd like to share is that the members as a whole don't have any drive as artists (Suzuki aside). When they talked about goals as a group it was always and only more fans and bigger venues. It was never about better music or more touching performances. So, after reaching SSA, they weighed the possibility of playing stadium sized shows and thought that they'd hit the top of their goal mountain. After SSA it would be "maintain this" or "try to descend gracefully" and seriously... if somebody asked them, "Now you've accomplished your goal of playing SSA, what's next for you?" and they answered, "We'd like to not lose our popularity quickly..." they'd get yelled at for days by management. When they talked about their goals as individual members Suzuki was the only one served by actually performing as an idol. Yajima and Okai were served only in that it was a reason to be in the public eye, etc. So if the group united has no reason to stay together, and most of the members have no reason to stay together...