Professor Higgins said in Pygmalion that every time a man opens his mouth he irritates someone. It goes for email marketing as well. You cannot satisfy everyone and you will therefore lose a certain percentage of potential subscribers by a casual, ill-judged phrase. In those cases where a few tweaks can lower that percentage, it makes sense to do so.
We need to reflect the make-up of society and especially that section of it we want to sell to. The percentage of those who do not consider themselves as male or female is small, probably no greater than 5%, but the number of people who will be concerned if you are not inclusive is higher, some suggest much higher.
The first question to answer is whether or not you need to ask for gender. It is not as simple as wondering if it alters the metrics. You might want to prove you are inclusive. If you work closely with public services then you might have to. If you don’t need to know, don’t ask.
If you want to know you will have to decide how to phrase the question. Having a free text box is one option, but this is expensive as it takes time to enter data manually, and not a little skill. It is, however, the most assured way not to upset anyone nor render yourself liable to criticism.
There is a much cheaper and as satisfactory way though.
Have a box with a menu. Have four options: Rather Not Say, Male, Female, Other. The final option will bring up a free text box. This covers all eventualities of course. Cost will be kept low as the vast majority will click either of the Male Female boxes. Those who opted out of defining themselves will probably be few, leaving <5% to cope with.
You must deal with them as specifics. You can’t lump them into one box as that would be dishonest.