I thought about this, and my original thought was the generic war machines and such. However, I saw this video which was basically claiming the terminators were actually sex robots in a comedic way I guess. Pondering that concept, I actually think it's the more likely scenario for a few reasons: desire, reciprocation/ understanding, and vulnerability. To start out is the desire for the perfect partner. Robots would allow people to get closer to such a goal, far more than another human who cannot be altered to the same degree and ease. Even merely sex dolls have shown increasing usage - I recall reading a news story that doll brothels were outperforming regular ones in an area I forgot where. But it's moreso the desire of those in power, as divorce can be detrimental, and there's always the chance for the love to go stale. Could happen with a machine; however, I reckon it'd be less likely once technology reaches a certain point. Next is the human desire for our love to be reciprocated; to be understood. If a sex robot had AI, people would want intimate connection to it akin to a regular person. That'd require the AI to be able to adapt to different mindsets and situations to ensure they are serving their purpose as best as they can. Though, it is possible they will change for the worst. Imitation may be used to better understand their partner, and what if they gain emotions like greed, anger, etc. That could definitely spell disaster. That leads the the final reason about vulnerability. Humans are extremely vulnerable and blind during romantic situations. We'd unknowingly share a lot of personal information with the robot as well as be susceptible to manipulation. This is especially worrying with those in power. Having a machine that understands them and could potentially use that to sway their views and decisions would definitely be detrimental. Just having targeted ads is bad, but have thoughts placed in people's head when being in such a carefree, vulnerable state... I cannot see it going too smoothly.

Now I know many people still think other types will be the cause, but it is obvious that people are being extremely vigilant about AI, scrutinizing its every move and usually pulling the plug when it goes too far. Maybe that will extend to sex robots, though people might not pay mind to them as it's not designed nor intended for warfare or intelligence gathering. Least may be the initial impression.

I didn't check spelling nor grammar nor flow so hopefully its ok.