They’re barely eighteen when Lucy starts to suspect that Lockwood’s considering marriage.
She tells herself she’s imagining it at first, because they’re young, and still a relatively new couple, and considering taking things any further at this point is nothing short of insane. She could be falsely interpreting things due to her own wishful thinking, but Lucy banishes that thought to the darkest corners of her heart, because they’re young, and still a relatively new couple, and the idea is insane.