Hi...
I'm pregnant with my first right now, but my husband and I do plan on having a big family. The biggest problem that we foresee is the age-factor on our ends. We want 3-4 kids, but doctors have urged me to have my children young due to very high risks of ovarian and breast cancer. I say this because otherwise, it would be nice to space things out, ya know?
Anyway, right now is obviously not the right time for you two to have another child because you are not in enthusiastic agreement over the decision. The best thing to do is compromise. The reasons that you don't want another child now are financial and freedom to travel. Perhaps you and your wife could strike a deal to re-evaluate your situation in a few years. At that time you could discuss your finances and what would need to happen so that you both felt financially secure with another child. At that time you may have had more of an opportunity to travel and maybe you won't even feel that is such a priority, anymore. Who knows...by that time you may be really wishing for a son, too, and willing to roll the dice, again.
Personally, in my situation, my biggest fear would be health/age-related. But, if you two are young enough, waiting a few years to make this decision is probably the best thing you can do.
Right now is not the right time, though. Leave the window open, don't make any promises, but help her to understand how important it is for both of your to feel enthusiastic about such a big decision to avoid resentment and further marital problems. Anyone on this site will tell you that the marital problems should be resolved before another one is introduced and I'm sure you already know that since you mentioned the current instability in your post.
Btw, how would your wife feel if you had another daughter? Would she still feel unfulfilled and want to try again for a boy? My FIL is #9 of 10 boys...a girl obviously wasn't in the cards for them!
Best wishes,
Smile