The whole idea is that if you entered multiple numbers, it must be because you want to be able to call/send texts to that contact at more than one number. If you want a default number for texts, add a contact for that person with the first name Text Default John or something like that, and only one number.
I have to thank JDMDATL, even years later for pointing out how boneheaded this reply is. Because unfortunately this is still a problem and this page comes up in Google.
No Rukbat, the whole idea for entering multiple numbers is so you have those numbers available IF you need them.
I have every single one of my family members landline (those that still have one) and work numbers stored in my phone. I could get stranded and not be able to get them on their mobile, before I pay for an uber or something that would be a good time to us those numbers. Or god forbid, what if there is an emergency that I need to talk to one of them specifically and their mobile is off as they're at work... again you should have the work number for that and where else should I store it? haven't you ever heard it's better to have something and not need it rather than need it and not have it...
That said I have NEVER USED ANY OF THEIR WORK NUMBERS, I shouldn't have Google ask me every (many) times which number to use since I only ever use mobile, however I should not have to delete their work number or make a separate contact. That's just foolish and I'm surprised this problem still exists years later.