First of all, it is important to understand these two verses in the context of the chapter as a whole. The laws from Exodus 21:33 to 22:17, for the most part, deal with paying restitution to one's neighbor to compensate for depriving him/her of some good.
- If a man digs a hole and doesn't cover it and his neighbor's ox dies by falling in the hole, the man must pay restitution for the price of the ox. (Exodus 21:33-34)
- If Man A knows that his ox is violent but nevertheless lets him roam about, and Man A's ox kills Man B's ox as a result, Man A must give Man B his ox as recompense. (Exodus 21:36)
- If a man steals an ox, the thief must pay back five oxen to his victim. (Exodus 22:1)
- If a thief doesn't have enough substance to make restitution for a theft, he must make restitution by being sold into servitude. (Exodus 22:3)
- If Man A's animal eats Man B's crops, Man A must make restitution. (Exodus 22:5)
- If Man A kindles a fire which destroys Man B's crops, Man A must make restitution. (Exodus 22:6)
- If Man A had custody of Man B's ox and the ox gets killed/stolen, Man A may or may not be required to make restitution, depending on the circumstances. (Exodus 22:10-15)
- If a man seduces a virgin and lies with her, he must pay a dowry and marry her if the father allows it. (Exodus 22:16-17)
Seeing Exodus 22:16-17 alongside the laws which precede it, one can make an educated guess that the purpose of the law is to mandate that restitution be made to the virgin (and possibly also her family) for seducing her.
See, the virgin was deprived of something when the man deceptively lead her to have s-x with him. Her virginity was destroyed. If she wanted to get married in the future, what man would want her, seeing that she isn't a virgin? This would have been a great hardship not only for her, but also for her father who wanted to see his daughter get married and have stable support. Who would provide for her? What if, out of desperation and poverty, she even went into prostitution in order to provide for herself?
Since it would be hard for the (former) virgin to find a husband in the future, and thus the seducer was depriving her of a husband, it seems that it would be fitting for the seducer to provide a husband for her, as a sort of recompense. He could do this by providing himself as a husband. (By the way, the seducer probably made an empty promise to marry the virgin if she would have s-x with him. This is a likely scenario envisioned by more contemporary seduction laws.)
Of course, the Mosaic Law foresees circumstances in which it might be better for the two not to get married. Maybe he's a real scumbag, and the father doesn't want his daughter marrying a scumbag. Thus, the choice is with the father, who will decide whether or not it would be helpful for the man to marry his daughter. In the case that the father says no, however, the man isn't "off the hook." He still has to pay money equivalent to a dowry as a sort of compensation.
I have explained the rationale behind this law. It is a law about restitution, not moral acceptability. Thus, there is no sin offering mentioned here. The chapter doesn't mention a sin offering required for theft either, but only mandates how much restitution is required for theft. Does this mean that theft isn't a sin? Of course not!