Leaving aside the moral arguments, which are not my concern, if this other woman likes to suck your penis, and you are able to give her the satisfaction she is missing from her husband, and I don't see that there is any other consideration accept for your feelings of guilt.
If you feel guilty, don't do it!!!
Have you considered why your wife doesn't want to suck your penis? Do you keep it nice and clean? Including drawing back your foreskin and washing it thoroughly. Some women have a bad experience of sucking a man's penis, because either he is dirty, or he gets too excited, and tries to stuff it down her throat. Other women refuse to do it because their partner will not go down and lick them. You cannot expect your wife to give you oral sex if you don't do the same for her. This may not be your situation, but if you have never given her oral pleasure, I suggest you try it, and see how she reacts.
The older generations have a lot to answer for when it comes to sexual practices. For God alone knows what reason, lots of mums and auntie's tell their younger generations that certain things in sex are dirty or nasty, and that "good girls don't do things like that." If your wife has been brought up in this manner, then changing her mind on something like oral sex is really a case of re-educating her. There are so many articles in magazines, and online, about the joys of oral sex, anal sex, watersports, you name it, it's all on there to read these days, why not try pointing in the right direction and showing her articles written by proper medical professionals. Buy her a copy of the book called the Joy of sex, or the Kama Sutra. If she looks at the Kama Sutra, she will see that such practices as oral sex both giving and receiving, have a long long history in India. The Kama Sutra was written a long time ago, and it shows that India was at least at one time far more enlightened about sex and people's sexuality than most civilisations that were its equivalent. Attitudes to sex in the Indian subcontinent really only changed with the arrival of the British, and particularly in Victorian times, with their moralising and preaching, and the underlying racism that suggested that Indians were somehow not quite as good as the white man. Of course it was rank hypocrisy, because most of the white men that came to India fell in love with Indian women, and enjoyed all manner of sexual practices with them. Not only that, but those starched upright men Saabs often took themselves Indian lovers, because their white husbands were so busy with their native girls (as they would have put it) the Victorian era was a hotbed of vice and depravity, while it mask raided itself as being stern and morally upright. It left an unfortunate legacy for people everywhere that the British went.
My grandparents came to London in 1961, my parents were both born in London, and so were my sister I. I have to say that things have changed very much for the positive in England. The English, and I am of Asian heritage, I consider myself as English, have a very free and easy attitude towards all manner of sexual practices these days, and I'm very happy with that situation.
For you, it poses a problem, because your wife finds the idea of sucking your penis to be distasteful, and therefore it leaves you placing an ever more unsatisfied desire to feel a woman's lips on your penis. In truth, this is just a symptom of a bigger problem in that your sex drive is much stronger than your wife's, and your idea of what constitutes an enjoyable and satisfactory sex life is not matched by your wife's ideas.Under such circumstances, you have resorted to things like phone sex, and actually meeting up with somebody, and enjoying a thoroughly satisfying time with her. The fact that she is now phoning you up, looking for more opportunities to enjoy sex with you, would suggest a couple of things: you are not a bad lover, as she is eager for you to share her bed again, and that you are finding it difficult to reconcile your guilt with your needs.
It comes back to what I said near the start of this reply, if you find your need is stronger than your feelings of guilt, then make an arrangement to see this lady again, but whatever you do, be discreet. If your wife will respond positively to you performing oral sex upon her, and you don't need to find anybody else, but if she continues to be something of a cold fish in bed, and I can't see what other alternatives you have, unless you want to condemn yourself to a life of masturbation and pornographic fantasies.
The other part of this equation is that we don't know anything about the woman with whom you enjoyed a lusty session. Is she promiscuous? In which case, you have to consider her sexual health, and your own. Is there a risk of pregnancy? Both sexual health and pregnancy can be avoided by the use of condoms. I would advise anybody who is having casual liaison is with people they don't know, or an extramarital affair with a woman who is promiscuous, that they must take sensible precautions. I can't even go into question about risk, with the possible consequences for you. Men have been known to kill both their wife and their wife's lover when they find them in "flagrante delicto" So your safety and that of the woman you are meeting have to be of paramount importance.
If you can solve your problems without resorting to meeting other women, whether it's seeing an escort, or having an affair, then I would do everything to remain faithful, but if you find yourself living an unsatisfactory life, then I don't think many people would blame you for taking the next step, outside your marital bed.
I hope I have helped in some way.