While you may not consider dating another guy, the same may not be true for the other woman. Being open to that possibility can only work to your advantage. By asking to be the only guy, it's as if he is stating that there is a difference between a M-F and F-F relationships. This could be interpreted as:
1) the love between two women is not threatening, because the relationship is not as important as that of a man and a woman
2) I don't want my penis/loving making to compared with another man's penis/loving making
The easy part of being poly is having the freedom to date other people, and to explore new relationships. The difficult part is accepting that you're not the only one who is entitled to that freedom. With a one penis policy, the guy is saying "I want the complete freedom, but I want add limitations to yours".
Although one aspect of a primary relationship involves financial entanglement, there is a danger that a third person can become financially dependent on the couple.
While I appreciate that you provided support out of love, it did inadvertently give you a controlling power over her life (although I suspect she was quite content to take advantage of your generosity). Such power can easily change the dynamic of the relationship. If you decide to pursue this type of relationship again, it is probably prudent to ensure that the woman is more independent, both for her sake and yours.