That statement is blatantly untrue!!!
I'm not saying that every single love hotel is perfectly managed, perhaps there are a couple somewhere with lax standards, but I believe you'd be hard pressed to find one (or perhaps you did and were unlucky).

I can also tell you that it is almost impossible to find double bedded (ie non twin-)rooms in any of the more upmarket 'regular' hotels on Xmas Eve, New Years Eve, or Valentines day. Why would you think that these hotels don't get used in the same way as any Love Hotel does?
I'd also like to point out that the kind of bedding used in Love Hotels is, let's say 'splash proof', and I would bet good money that it is a lot more hygienic than most beds/futons used in more conventional hotels.

Of course, you may have meant the word 'dirty' in a more moralistic context - OK fine. Nothing I could say would make any difference if that is the case.

There are many downsides to using love hotels, but dirtiness isn't one of them! (They rely on repeat business for survival, and it's also generally the females who have the say in which hotels are frequented. I'd ALWAYS put more reliance on a Japanese lady's judgement of hygiene standards than a businessman's.)