I thought I'd through these interesting facts about male/female ratios. By the way the census department publishes these facts in their age reports.

At birth males outnumber females 105 males per 100 females. Nature does this because males have a higher mortality rate than women at almost every age. Around 27 this evens out so that there are 100 males per 100 females. However, after this evening out the mortality issue continues such that later in life there slowly continues to be less males per and more females. This is most evident at the disparity in average ages for death, women being 78 and men being 70.

Which depending your age explains alot of the dynamic you are seeing in the changes of your dating pool.