OK. I'm going to say something that is advice I would give to any wife, assuming that until her husband has decided without a shadow of a doubt that he is TS, then there is a chance that he might not be ... even though we are in the TS section of this forum. It is rare that a wife posts in her husband's thread in the TS section, but it is what it is.
I cannot emphasize this strongly enough, but whether Steph is TS or not, she does need to leave her closet. Non-TSs leave their closets too, for example my SO is not TS (not planning HRT or SRS), but s/he is more than just an occasional CDer who is happy putting on girlie clothes and staying home. S/he needs to interact with others as herself. This is a natural progression for MTFs who are gender non-conforming, and so he has constructed a life for herself where she does this fluidly - she goes out on a regular basis, while at the same time protecting his male life since most people do not understand this - and this means looking the part: i.e., not having male tell-tale signs such as hairy legs & chest, and a beard shadow. A TS wants the same thing (no body hair, no beard), but a TS also wants to not live as a man at all, ever, and eventually wants to modify her entire body as well (breasts and a vagina).
If my SO were in a relationship with a GG who put the brakes on his need to express herself fully, this would be unbearable to her/him. S/he would not be happy and he would need to find a way to express herself no matter how much he loved his SO. Just because s/he is not TS does not mean that feminine gender expression is an option, nor is it a fetish or a "hobby". The only way that s/he can maintain a healthy balance with his gender duality (for lack of a better word), is to have the ability to express her femininity whenever s/he feels the need to do this, which works out to a few times per week. S/he has a rather busy schedule and s/he is also invested in his career and other things that he is interested in.
This is really hard to explain. :p
So anyway, you have choices:
1. Not accept Steph's needs, which might (or will?) cause the two of you to break up, or cause Steph to stay in the marriage but be deeply unhappy.
2. Accept Steph's needs to see where this will all lead.
If you choose option 2, one of two things will happen:
A. Steph will cease to be frustrated with his current inability to be herself and she will eventually find a balance given your needs too. There will be a separation between male and female lives, since this is the only way that someone who is not fully TS can live in our current society.
B. Steph will eventually decide that she is TS and will want to live full time. If this happens and you are not OK with being in a lesbian relationship then you will divorce.
So. If you do want to attempt saving your marriage, the only viable option is (2A), because each of options (1) and (2B) will spell the end of your marriage. This will mean a great deal of stretching and deconstructing gender (moving away for the idea that all people are either fully male or fully female) on your part and I don't know if you can do this or not. But, it is necessary if the two of you will find a way to stay happily married, where both your needs will be met.
Blah! I hate typing something like this in the TS section, because I know that many TSs do not believe in the idea there are varying degrees or different types of gender dysphoria.