In the case In re Paternity of JLH, 441 NW 2d 273 - Wis: Court of Appeals 1989, the court explicitly refused to say one way or the other:
we do not decide whether the defense of nonconsent is available to a putative father in a paternity proceeding.
This was because they found that, although he was only 15 and the criminal statutes indicated a presumption that a 15 year old cannot consent, this presumption does not apply in a civil case and his actions showed that he consented.
This case was cited in the Wisconsin Annotated Statutes, so it would seem that no later case has decided the matter. Presumably, then, you would have to go to court to find out - at least in Wisconsin.
Although the court did not decide that key question, I get the feeling they would have decided it against him:
We reject appellant's argument that his paying child support to L.H. would permit her to benefit from her crime. Even assuming that L.H. criminally assaulted appellant, child support is paid to benefit the child, not the custodial parent.
They also said:
Whether he is the father is a biological fact. Whether the fact does or does not exist cannot shock the conscience and has nothing to do with fairness. This is true no matter how conception occurred.
Also, in the case In re Paternity of Derek S.H., 642 N.W.2d 645 - Wis: Court of Appeals 2002, the court decided that a man must pay child support even though a jury found that he did not consent. That case, however, is officially "unpublished" and is not binding precedent, and involved finding that the jury's verdict on consent was merely advisory (since the law says they're only supposed to determine paternity) and the trial court was free to reject it.