The question, "Is sex in itself considered bad?" is a very different question from "Is sex in itself "bad"?" You distinguish between these two questions, but from the other answers provided within this post, it seems that most people are more interested in the latter question. After all, the question "Is sex in itself considered bad?" can simply be answered by saying the following: it depends on the time, place, culture, and relevant contextual implications that surround you at the time you ask it. Certain cultures might forbid sex to enforce chastity upon the women in their society, due to certain values of feminine virtue; some might do so in order to emphasize the sanctity of marriage itself; and others might suggest that premarital sex is perfectly natural. Most modern societies (though unfortunately, not all - and not all successfully) forbid the sexual exploitation of minors, non-consenting individuals, and other vulnerable agents for obvious moral reasons. Even so, these prohibitions are technically more concerned with stopping coercion, exploitation, violence, and human rights violations, and paternalism, not sex in-and-of-itself when considered under the most favourable circumstances (i.e. between two consenting adults).
The original poster wrote that the question "Is sex bad?" is opinion-based, which is why you don't want an answer to it. I disagree. The question "Is sex bad?" seems (to me) to place emphasis on the activity itself, while the question "Is sex considered bad?" seems entirely based on the opinions of others. The issue I take with the question "Is sex considered bad?" is, as with every activity in life taken at face value, sex can be associated with certain observable and/or quantifiable "benefits" or conversely "risks", depending on ones perspective on the matter. For instance, the importance of the biological act to the preservation of the human species, to the reinforcement of the human psyche (self-confidence, social relationships, trust-building, etc.), purported health benefits (increased production of oxytocin and antibodies, lower blood pressure), as well as the subjectively pleasurable benefits of the act itself, can all be said to be the beneficial aspects of sexual intercourse - depending on your point of view. But there are also detrimental aspects associated with sex that seem just as convincing - such as STI's, teenage pregnancy, the exploitation/coercion/violation of vulnerable or unwilling individuals (which, statistics tell us, are real and ongoing issues worldwide), and so on and so forth.
But is it the act itself that causes these issues? Could one not also argue that it is rather the moral agents or associated environmental factors behind the act that can turn this otherwise neutral activity into a bad one? Sex might just be a fact of life - but AIDs, love, rape, and long-term partnership are not the intrinsic properties of the act itself. As these relations are things commonly associated with this act, it is easy to see where someone might say that society places normative, moral, and legal restrictions on this act in order to prevent some of the risks associated with it. Approached in this way, the answer to the question, "Is sex considered bad, and why?" is, "Sometimes. It depends."
But without these extra considerations, it's even harder a question to answer. Does anyone ever really consider sex without these related aspects (like disease, promiscuity, reproductive value, health benefits, etc.) attached to it? Without these considerations, sex becomes (at its most basic) a deeply intimate act which is valued in dramatically different ways by most people - and therefore it does not have a universally agreed upon value attached to it.
It is interesting to note that, despite what people might argue concerning things such as "Christian guilt" and other Western-based notions of sin and sexuality, that the issue of sex and its intrinsic meaning is something that all cultures have a deep and ever-evolving history with. I personally approach this question from the perspective of political philosophy, which causes me to ask how the induction of a social contract might influence the assumed need for a society to create a unified moral value with which to associate and regulate this act. After all, this is something highly personal that we're talking about here. When you engage in this activity, whether you think it should be reserved between people in love or performed for casual pleasure, it is still an instance in which people must lay themselves bare before someone else. We all must engage in this act with our own internal interpretations of it in mind. It might just be that the conflicting values of this act affect people more than, say, traffic-rules do. Few will argue for very long about something inherently obvious and distant from their personal lives, like the need for seatbelts in cars, or the requirement that no one should drive under the influence. But if people believe that love should be a requirement for sex, and other people think that to be an old-fashioned notion, then they might feel threatened and uncomfortable with the imposition of a stranger's values on a very personal part of their own lives. Similarly, it might have once been seen as helpful, from an economic perspective, to enforce ones citizens to be monogamous, and to make infidelity illegal - as average household incomes would rise, children would have guaranteed parental units to care for them, partners could help to ensure the health of their spouse, and so on and so forth (not a view I endorse or reject, just an example of a particular line of reasoning).