Alright, let's test the hypothesis. Let's say there is a man who was a former child molester. He's repented, done his time and is seeing a woman. She is not allowed to reject him for his past, right?
To be fair though, that’s not mere promiscuity, as
@Paidiske pointed out.
In Orthodoxy, we do stress forgiveness, although I see nothing wrong with someone not wanting to marry someone who had previously lived an extremely immoral life, for reasons of personal concern, but if that person has repented there are also surely cases where such marriage would be positive.
This is particularly the case if the person repented and was baptized into the Church, in which even canonical restrictions against Holy Orders are set aside (which is how St. Moses the Black and more recently, the saintly Fr. Seraphim Rose, were able to be ordained, despite having committed sins which would have disqualified them according to a strict reading of the canons, had those sins occurred after reception into the church, at least.
If we look at the canon law of the early church, the early church would not receive a prostitute or a pimp, but if those persons repented, then they would be received by the early church. They could simply not join Christianity while maintaining their employment in prostitution, as it were. This canon, which is among the Apostolic Canons, also applied to Gladiators and to teachers of Pagan sophistry.
So if someone has repented from engaging in prostitution or other forms of sexually immoral employment, and been received into the church, or re-received had they apostasized, then they ought to receive the clean slate that forgiveness promises.
In the case of a child molester, they fall under the much more severe canons of the early church that apply to those who engage in arsenokoetia, and there are specific canons that apply to men who have harmed boys in this manner, by engaging in the common Greek vice of pederasty, and these canons are always applied with severity, so that someone who has done such a thing can never receive Holy Orders, for example, because they cannot be trusted with any position in the church.
But there is a huge difference between someone who is sexually promiscuous or who engages willingly in prostitution or obscene forms of dance, and someone who is a child molester. It is not an apples to apples comparison.
We must also consider, and I believe our friend
@Paidiske has pointed this out, that many people who engage in prostitution are literally forced to do so, for they have been the victims of human trafficking, which is to say, actual slavery. In such cases as that we cannot say that any moral culpability attaches to the acts that they have been compelled to engage in, and it is the responsibility of the Church to help any people who have been trafficked, who it has the opportunity to help.