Thought I'd replied but may have clicked the wrong button... if the post is caught in moderator purgatory sorry if this ends up a doubled post.
For me I certainly find that my libido spikes each summer, generally making me a bit crankier due to the distraction of wasted thoughts and neuropeptides.
The irony though is I know from when I did date that my ability to "perform" tanks in the heat, have a pretty poor tolerance for heat frankly.
Regarding the suggestion they get fewer punters one possibility is many are out enjoying the view of women in their warm weather attire, I know studies have shown an increase in road traffic accidents in summer due to that.
Or maybe they're having to bring their families for a day out instead of using the cover story of "going to the hardware store".