To give some context, the age of consent pertains more to the laws regarding rape. It does not really have anything to do with the age at which young girls became sexually active (or were even considered marriageable) in the 19th century or earlier, in Delaware or anywhere else.

Most early US laws were inherited from English Common Law, under which rape was a rarely prosecuted crime with even fewer convictions. A woman who had "given her consent" could not be said to have been raped under the law, even if such "consent" was coerced through violence, force, or threat of same. Most rape cases pertained to indisputably vicious and brutal attacks, often resulting in the death, near-death, or maiming of the victim.

The exception was the case of extremely young girls, whose virtue (virginity) was considered 'particularly precious' under the law, and was thus protected. That's what "age 7" pertained to here. (Other US states had 10 or 12 as the age of consent at this time.) It's the law saying that if someone is proven to have penetrated a girl this young, then that person is unequivocally guilty of committing rape. The victim legally could not consent, period. There are no mitigating circumstances or arguments to be made in this case.

Today the law is much more expansive when it comes to crimes of rape, sexual assault, and the abuse of minors, including many different definitions and degrees of severity, but there was a period of time before the late 19th century when only a small handful of crimes of this nature were defined under the law. As such, the strokes were painted very broadly. Reforms to protect children from sexual crimes began in earnest in the 1880s in the US, and continue to the present day.

As a small note, Delaware's age of consent had been raised to 15 by 1890.

Jailbait by Carolyn Cocca is a good read on the history/politics of statutory rape law in the US. You can read chapter1 for free here (warning,pdf).