There's nothing ridiculous at all about the explanation. Your book is right - 'every', like 'all' and 'any', is for three or more elements. If there are only two of something, you cannot use 'every'. If there are only two, you can use 'both' and 'either' - which are the dual equivalents of 'all' and 'any' - or you can use 'each'. 'Each' can be used for any number of objects. In fact, 'each' is the best word here. It very clearly means a kiss on one cheek, followed by a kiss on the other. A kiss on 'either cheek' is possible, but this is ambiguous. 'Either' can mean 'one or the other'. For example. 'Should we kiss each other on the right or the left cheek?' 'Either'. This means 'either the left or the right' - but not both. A kiss on 'both cheeks' is possible, but we tend to use 'both' when we mean 'both at the same time'. For example, 'She took both my hands in hers' - this suggests simultaneously. 'Kiss every cheek' isn't possible, because we have only two. Well, in fact, we do have four cheeks in total - two on our face and two more..... elsewhere on our bodies. So if you say that someone greets another person by kissing 'every cheek', this is actually quite funny and a little bit rude!

Your book is correct -- "every" should not be used with fewer than three things. English has singular, plural, and sometimes, dual number. You could say "both cheeks," if you'd like. Another example of dual number is "either." I hope this helped you.

We use each to refer to individual things in a group or a list of two or more things. It is often similar in meaning to every, but we use every to refer to a group or list of three or more things. so 2 points. 1. if you say every in this context it makes me think they kiss all 4 cheeks i.e. 2 on face and 2 on bottom. 2. each is better because the action is done to the individual cheeks separately. every is used to refer to the group, each refers to individual items in the group.