The first time that you come in contact willingly with the cum of your wife's lover is in my opinion quite milestone in the cuckold dynamic. There are the milestones for the wife, for the cuck, the lover and for the with each others interacting people. At first when my girlfriend was cheating on me and that I did not know it and that she came home after she had been seeded by her boyfriend she always refused me to go down on her. She pushed me away and did not want to have sex, or made me cum by hand to satisfy me and keep my attention from her pussy.
In a cuckold-dynamic there are different stages, from whom there are some of moments of no return. E.g. the moment she tells you she is cheating, or that you discovered it, the moment she knows that you accept that she has a lover, the first time she goes bare with her lover and that she tells it to you or that you discover it, etc...
The moment you suck her lover's cum from her pussy willinngly where she realises that you fully want to do that, is one of these milestones were you also accept apart of the fact that she accept that her lovers owns her in an emotional, sexual and psyhological way but also in biological way by spreading a part of his body's own fluids and DNA in her. For the cuck there are also these personal milestones as the emotional, sexual and psyholgical acceptance but the first time that you ingest the semen of her lover it is amilestone for you, but also for her when she realises this. On that moment you also acept her lover in biological way by taking in a part of his body's own fluids and DNA. The it is quite difficult to reverse your role to an Alpha-male again..