Maryland appears to have no law criminalizing mere possession of obscene matter except in the case of child pornography, in which case mere possession or even viewing on the internet may be prosecuted. There are laws restricting or prohibiting the production, advertisement, sale and distribution of various types of obscene images, but not mere possession.
This is an extortion scam which may have infected your computer with a virus in the process. Run an anti-virus scan and Google/Bing "pornography extortion scam" to learn more.
This answer is offered as a public service for general information only and may not be relied upon as legal advice.
What you've described is a virus and/or an extortionist scheme. Law enforcement does not contact anyone via "pop up messages" on their personal computer. Whatever "warning" spooked you was generated by a third-party for the purposes of gaining personal information and/or money.
The First Amendment protects sexual and pornographic expression. "Speech" (which is used broadly in constitutional discussions to mean any kind of expression) that falls within those categories cannot be criminalized. However, "obscenity" is not protected by the First Amendment. The Supreme Court, in Miller v. California, defined "obscenity" as speech that:
(1) appeals to the prurient interest (judged by an average person, applying contemporary community standards);
(2) depicts or describes, in a patently offensive way, sexual or excretory functions; and
(3) taken as a whole, lacks serious literary, artistic, political, or scientific value.
See, Miller v. California, 413 U.S. 15 (1973). Further, the possession of pornographic material has been upheld as constitutional by the U.S. Supreme Court. See, Stanley v. Georgia, 394 U.S. 557 (1969).
This is a scam where the website sends a warning notice to the viewer. The follow-up in some cases is a demand for money. Ignore the warning and reset your computer.
But as to your question, is viewing sexual contact between animals and people illegal in Maryland? The answer is no.
That being said, pornography has the potential of becoming addictive. And as with any addiction, it takes increasing levels of stimulation to create the same of arousal resulting in the brain's neurotransmitters releasing dopamine, serotonin and other chemicals. In other words, many people who eventually begin viewing child pornography (highly illegal) start with “regular” pornography. While many people view adult pornography without progressing to anything illegal, it is something to keep in mind. Good luck.