If your guy doesn’t like long hair...

I'd like to start off with a disclaimer:
The following is written in good humor. It’s an attempt of giving advice and asking to see things from his perspective while doing it in a teasing tone.
I asked my hubby to read it and his reaction was calling me a “big meanie” while hugging me, so I guess an easygoing man can see the intended humor in it.
Of course I am not a psychologist, neither a couple’s therapist but I am a girl with knee length hair and I’ve had a couple of relationships since I started actively growing out from bald and I think I have a good understanding of psychology.
I will only cover "normal" men, as in no asshats who object to your hair just to control you. Unfortunately I dated a selfish drama queen like that for 1½ year. Strangely, I don’t even think he really disliked the hair, he just wanted me to change it because it mattered to me and he wanted to manipulate and control me. Anyways, jerks of that caliber will not be covered here.
All people, male or female, has preferences in the opposite sexes appearance. Ask yourself if your partner has your perfect fantasy appearance. He doesn’t, does he? But you love him just the same and he is the sexiest male in the world to you, isn’t he? Surprise, men aren’t really different in that way. The only difference is that we women tend to take it very, very personally. It’s a bad habit that hurts both ourselves and our partners. Make peace with the fact that he may have a different type fantasy girl but that he still loves you. Either that or stop wasting yours and his time.
But like all sentient animals, men can be trained...
All men just want you to be happy in your relationship and they can be guided gently if you know how to handle them and when to reward them.
But first, start off by asking what you really want from him when it comes to your hair. Have a defined, clear objective in your head on where you want to end up with him. Do you just want him to shut up about voicing his opinion on your "too long" hair? Do you want him to help you care for it and admire it? Do you just want him to express simple opinion such as liking one bun better than another?
Positive and negative reinforcement
Why yes, we are actually going there, back to basic psychology. Don’t underestimate how powerful this is. Reward the behavior you want by expressing your joy immediately after he acts or speaks to reinforce it. A simple "Aw, honey, you’re the sweetest!" will do. He will want to repeat what he said or did to see you beam in happiness. Something similar to "That hurt my feelings" is a powerful negative reinforcement. No man wants to hurt your feelings.
Don’t go where he can’t win
If you already know that your man has a different preference than your hair, don’t ask for opinions or questions when he can’t win. It’s cruel to him and puts you both in a lose-lose situation where he either has to lie to not hurt your feelings, or express his honest opinion and see your hurt face. If you want to ask his opinions on your hair, keep it to something neutral and specific like "What do you prefer? This Ficcare or Flexi in this bun" instead of asking him what he thinks of a haircut, colour or style.
Simplify your communications
I find in general it makes life much easier when I simplify my communications with men. To keep it hair-related, make your own decision that you want to wear something green in your hair and then ask him if he prefers the Ketylo or Flexi in your bun. Don’t expect him to be able to go into a dialogue about the pros and cons about an up do. Make your own hair decisions and then involve him in simple decisions such as colours of hair sticks or style of clips. 

Show him his opinion matters
He might feel "overlooked" when you ignore his opinions on your hair (That silly male pride...) but show him that his opinions matter for other things that matters less to you. Ask him what pants you look the best in or ask what nail polish he prefers. If you notice him admiring something, say, a sexy pedicure, do them as a surprise for him. Show him that you pay attention to his preferences and tastes. This also acts as a distraction to keep him off the hair topic.
Unfortunately, we live in a society where “long hair” is around bra strap length and “very long hair” is waist. Longer hair than that is seen as abnormal and often causes weird, hostile reactions from people (The abundance of “My friend/mother/aunt/boss/etc. said something mean and hurtful about my hair!” threads found on most hair forums speaks volumes) Naturally, a partner won’t be different in the perception of “normal”. Even if he adores your hair, there is no reason to tell him the details of how long ago it has been since last you used shampoo and how half your kitchen supplies double as hair treatments. It might be pushing the “norm” too much to make him feel comfortable. Save your joy of the newest avocado-honey treatment for sharing on UTT with kindred spirits who enjoy it too.
As a matter of fact, generally it seems to me that men aren’t interested in your grooming habits and prefer to believe you roll out of bed looking gloriously sexy and not know how many hours you spend on exfoliating, waxing, doing pedicures, mud masks, digging out blackheads, bleaching unwanted hair, shaping eyebrows, makeup and so on…  

To end this with an anecdote...
When I had hair to just my shoulders, my boyfriend thought it was "long enough" and that mid back was "creepy". When I was at mid back/waist-ish my boyfriend thought that it was "long enough" and classic length hair was "creepy". When I hit classic, the boyfriend then thought that was "long enough"...
I guess what you do or have, will always be measured as the "norm" and anything out of that will be seen as "abnormal". If you just let your hair grow (And let’s face it, it doesn’t grow fast enough to take anybody by surprise...) he will eventually get used to it. Bad pun intended: It will grow on him.
Luckily now, I am married to a man who loves and adores my hair. He likes to pet it and smell my braid (Weirdo) but he doesn’t get involved in the handling and care of it, which is more or less what I wanted from him. As simple as I made everything sound, I’m still working on teaching him that he isn’t welcome in the bathroom when I handle my hair and put it up though! Maybe he will learn when I finally one day accidentally elbow him in the face...?
Good luck with hair-training your man!