We already answered your same question at length. You are simply asking for an indeterminate, life sentence in prison with a bunch of other prisoners who beat up and defile people who are convicted of child sexual assaults. Simply do not do this.
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Important disclaimer: I am licensed to practice law in Oregon, not Colorado, and so can't give advice on Colorado law. That makes this just a general discussion of legal principles. If you have specific questions, you should consult with a Colorado attorney.
That said: This is a bad idea on so many levels.
First, most states have laws that prohibit sexual contact with persons below a certain age (the "age of consent"). The penalties, to the adult in the relationship, can be quite severe. Not only can it lead to jail time, many states have instituted permanent sex offender registrations that require persons convicted of such crimes to notify their employers, neighbors, and anyone else who asks that they've committed a sex crime. With the possible exception of "terrorism" (whatever that means), there is no crime today with a greater social stigma than pedophilia. The mere suggestion of it ruins lives. The public shame and condemnation is so great, that a 20-year-old man would be a fool for even having a non-sexual close relationship with a 15-year-old, purely due to the risk that people would infer that he was sexually interested in children.
If a 15-year-old gets pregnant, then her legal rights are not much different than an adult woman's. She cannot be forced to have an abortion, either by her parents or her partner. She also cannot be denied the right to an abortion if she wants one, though it can be very difficult for teenagers who live with their parents to overcome the parents' wishes in these matters, especially if the parents are dogmatic, strict, or abusive. If this is an issue, the thing to do is to look up your local Planned Parenthood chapter. There may be other local resources as well. Google can help with this.
Assuming that the 15-year-old chooses to keep and bear the child, then she has custody rights over her child. The father would in theory have custody rights too, though it is entirely possible that if he ever tried to assert them, he would be denied because of his status as a sex offender. Any attempt to assert custody rights would necessarily require him to admit that he'd had sexual relations with an underage person, which, as stated above, would be an admission of a serious crime. The state might even terminate his rights outright.
Regardless, he would still be obligated to pay child support to the mother - though his ability to get a job, as a sex offender, would be severely curtailed. He would face a mounting, impossible-to-discharge debt, dwindling employment prospects, and additional criminal charges for failure to pay support if he couldn't make it. His life, in short, would be utterly ruined.
The mother, meanwhile, would be left to care for the child, probably without a lot of help from the father. It is possible that the state child protection agency would threaten to take the child away if she let the child have contact with the father - because he'll be a sex offender. In these situations, the young mother usually leans on her family a lot for help. But this can be difficult, again, if the family has "moral" objections to her pregnancy (by which I generally mean, religious objections, since that seems to usually be the source of the objection - though there is nothing moral about condemning young people for sexual activity, especially when it results in rejection and suffering for a new mother and an innocent child). It is very difficult to finish high school or college while also caring for a baby, or to maintain a job, so the mother's future employment prospects are likely to suffer quite a lot as well. This will make it, in turn, a lot harder to provide for the child. A life of poverty or dependency would likely follow.
In conclusion: Don't do it.
It is my personal belief that there is nothing wrong or evil about sexuality, even for minors. But it should be explored safely and responsibly, in an atmosphere of respect and equal power. This would not be that.
Please read the following notice: <br> <br> Jay Bodzin is licensed to practice law in the State of Oregon and the Federal District of Oregon, and cannot give advice about the laws of other jurisdictions. All comments on this site are intended for informational purposes only, and do not constitute legal advice or create an attorney-client relationship. No posts or comments on this site are in any way confidential. Each case is unique. You are advised to have counsel at all stages of any legal proceeding, and to speak with your own lawyer in private to get advice about your specific situation. <br> <br> Jay Bodzin, Northwest Law Office, 2075 SW First Avenue, Suite 2J, Portland, OR 97201 | Telephone: 503-227-0965 | Facsimile: 503-345-0926 | Email: firstname.lastname@example.org | Online: www.northwestlawoffice.com