For the record, I don't actually think that gender change is easy. Gender change is one of the hardest things a person can do emotionally, physically, socially, financially... However, as the partner I've found it far easier to adjust to my spouse's transness than to a non-monogamous lifestyle.
My grieving period after Jake came out as trans felt like it lasted about three weeks. If we were still monogamous, I can almost guarantee that I would be still grieving now (8 months later). Polyamory strips away the layers of internal conflict that come from relying on a trans wife to meet all my romantic and sexual needs. With those aside, I am free to love Jake unconditionally. I don't care what gender she is, so long as she's the same hilarious, irreverent, philosophical, intelligent, loving, cuddly person at her core. All the rest of the stuff---body, mannerisms, voice---seem superficial and inconsequential so far.
Instead of grieving the loss of my husband with each transition milestone, I celebrate the wife that she is becoming. I can be her biggest cheerleader without all the reservations about what this means for our marriage, what this means for my sexual identity, and oh-my-god will I never have sex with a man again. Polyamory makes the gender transition easy.
Non-monogamy on the other hand is difficult:
It's not difficult in an angst-ridden daily struggle kind of way. Most days I am overflowing with love and gratitude. I frequently find myself bursting into spontaneous happy dances, giggling in bed, and commuting to work with a giant grin on my face.
It's not difficult in a test-our-relationship kind of way either. I've seen other poly couples experience a lot of pain and strife when they first open their relationship. There is something about polyamory being the singular solution to us staying happily married that makes it easier to adapt.
Non-monogamy is simply difficult because it's different. Any big life change, no matter how awesome, requires a period of adjustment.
These are a few rules I've developed to cope with the change:
1. Give myself permission to feel. Occasionally I get jealous, insecure, sad, lonely or scared. So much so that I have a little cry about it. Then I start crying harder out of self-loathing for having these feelings, which I think are irrational and not aligned with what I want to feel. Then I feel guilty for crying so hard, because I think it sends the wrong message to my partners about how I support their poly happiness. So pretty soon I'm bawling because I hate myself for hating myself for crying.
I have to stop beating myself up for having feelings. I am human. Specifically a human who has been conditioned for 30+ years to believe that you should only be in love with or physically intimate with one person at a time. That is a lot of monogamous mindfuck to overcome. It's not going to happen overnight, no matter how much I embrace my new lifestyle, or how perfectly it's been working out so far.
2. Own my emotions. Giving myself permission to feel is not a license to lash out or use my emotions to manipulate others' behavior. Since crying can often be interpreted that way, I am explicit with my partners:
"I am only crying because this is still a big adjustment and I just need to have a good cry about it. I do NOT want to you to [skip your date, not spend the night, stop seeing the cis* girl, etc.]. That would only make me more upset for infringing on your happiness. All I want is for you to have a good time and for me to adjust to it. I am really happy for you even though it doesn't look like it right now because I can't stop fucking crying."
3. Ask for what I need. Sometimes it's reassurance that my partner will still want me even if he finds someone else amazing (I know this already, but it's sure nice to hear). Sometimes it's a small adjustment in behavior such as checking with me before confirming date plans so we can carve out enough us-time during the week. Sometimes it's nothing other than permission to cry guilt-free. Sometimes it's nothing at all.
Why live this way if it's so challenging?
Well, it's not a fraction as challenging as trying to live monogamously with a spouse whose gender conflicts with my strong sexual orientation. And it's certainly not as challenging as getting a divorce when we feel like we're each other's safe foundation in life.
Plus, the benefits are tremendous-- an abundance of love, sexual adventure, personal growth, an ever-expanding extended family...
And endless possibilities.
* "Cis" or cisgender is the label for someone who was born as their current gender, i.e. I am a cis female. It was a bigger adjustment for me to see Jake with another cis woman than it was for me to see him with a man or even a trans woman.