Is it sin to marry a woman who has committed adultery? She is not married and I'm willing to forgive her and start afresh with her.
Technically, you have nothing to forgive her. When she committed adultery, she sinned against God, against the man's wife, and against herself. "Flee sexual immorality. Every sin that a man does is outside the body, but he who commits sexual immorality sins against his own body" (I Corinthians 6:18). You weren't in the picture.
There is no requirement that a person has to marry a virgin. Hosea was told by God to marry a prostitute.
"When the LORD began to speak by Hosea, the LORD said to Hosea: "Go, take yourself a wife of harlotry and children of harlotry, for the land has committed great harlotry by departing from the LORD"" (Hosea 1:2).
The word translated "harlotry" is the Hebrew word zanuwn. It means to commit fornication. It is the word used of a prostitute -- someone who is paid for sexual acts -- but it isn't limited to such. Any one who had been sleeping around would be described by this word.
The phrase "children of harlotry" can either mean that Hosea was to take in the children she already had because of her harlotry or that he was to accept the fact that his children would have the stigma of having a mother who was known to sleep around. It would not be unexpected that marrying a woman known to commit fornication, people would wonder how many of the children would actually be Hosea's.
What should be noted is that despite the life of trouble God just assigned to him, Hosea did as the Lord asked, without complaint.
This is an extreme example, but it does prove that marrying someone who is not a virgin is not a sin. However, it is a warning that it could lead to difficulties. The past isn't nearly as important as the present. Has she changed? Does she despise her past sins and regrets that she committed them? Does she now stand against such sins? (II Corinthians 7:10-11). If she has repented of her sins, radically changing who she is and how she behaves, then what she did in the past no longer makes a difference.