I've been volunteering in many aspects of pro-life activism over the last 15 years and I have never heard of encouragement to "keep the baby" being intended or interpreted in opposition to adoption. Not once. Also from what I have seen, it is the previous sexual activity regardless of the presence of a child that makes "these young women less marriageable." Your analysis might be confusing the causation behind these trends.
You think the presence of a child is not a hindrance to finding a good husband? Most men I know want to have children of their own not raise someone else's. A woman's virtue may at one time have been THE deal breaker but I think, unfortunately, that is not essential to many young men today.