Morally, Ethically, Spiritually, it is *ABSOLUTELY CORRECT* to love more than one woman at a time. In fact, ideally, one should (if one is spiritually evolved and willing enough), love everyone at the same time. The entire Universe. This includes members of the opposite sex.
And bear in mind that every relationship is different, hence the love you have for different people will naturally be expressed differently, uniquely (since every relationship is unique). Afterall, you don't love your old bestfriend/buddy or your dog in the same way you love your girlfriend/wife, do you? (The topic of Brokeback Mountain will be discussed another time, on another thread).
You see, every soul (all right, the majority of people on the planet) have lived many, many lifetimes in the past (for some, even on other planets), and hence, each sould would have had plenty of opporunity to have forged intimate relationships with many different souls. Yes, you have been married to many different women/men/souls in your many past lives.
Therefore, when one man is emotionally torn between two (or more) women, it is a safe bet that karma is at work. That is to say, the emotional energies and love from your past life relationships with both women, are brought into your awareness now, resulting in a struggle.
But bottomline is, it's OKAY (ie. spiritually, morally, ethically, karmically) to have feelings of love for more than one woman at a time. If you deny yourself such feelings, you are just lying to yourself, and surpressing something beautiful you have for the other person - love.
Having said that, whilst physically incarnated on Earth, humans have developed a cultural system, such that for every lifetime, humans (as a whole) have chosen that there be only one married mate. That is to say, the institution of marriage is an exclusive one. One man shall only be officially married to one woman, and vice-versa.
As such, it will be perceived emotionally by your wife as an act of emotional betrayal if you have sex with other women outside your marriage. And the emotional wound and (sometimes irrepairable) damage to your marriage and relationship with your wife, is by itself a potential consequence and karma. (Karma isn't punishment, it's simply cause-&-effect).
Therefore, ideally you can either continue to emotionally love both women, but not have sex with the woman out of your marriage so as not to betray your wife (afterall, you did choose to marry your wife, so it's a matter of ethics for you to uphold this marriage, which is a lifelong contract, but only for this lifetime).
Or if you find that circumstances are such that your current marriage is already a limiting, unhappy one (for both husband and wife), and that if you were with (eg. married) to this other woman, it would be better for all 3 of you, then the correct thing to do would be to seek a divorce and separation. But do not do this for the wrong or misguided reasons, for everything has its ethical implications, emotional consequences or karma (which I emphasize again, it's not 'punishment', it's simply 'consequences').
If you require further personalized advice from a higher level, I recommend the Hilarion Reading. Submit your (and your wife's, and your girlfriend's) full name at birth, date and time of birth, location of birth, and Hilarion will zero in on your soul and make a connection, and give you advice regarding your questions.
Such questions in this context (of torn between another woman and wife), might be along the lines of "Please describe my relationships, on an emotional and karmic level, including past life relationships, with both this woman and my wife."
And of course, "I'm at a loss as to what to do about this. Can Hilarion and my guides & helpers please advise me to understand more clearly my situation, and advise me on the highest course of action for the highest benefit of all involved?"