Opportunistic infidelity is getting involved with someone else because it presents a good opportunity. A benefit of some sort of a career advancement. For instance, Paula Broadwell got involved with General Petraeus when she was writing his autobiography. Knowing that by getting involved and getting intimate with him her autobiography would be more popular. She would write herself into his autobiography.
Opportunistic infidelity often presents itself in career advancement cases. When someone knows that by getting involved with their boss, they can get an advancement or promotion, they're often very likely to take advantage of that. It may also present itself in cases of financial opportunity, knowing that that person may in some way support that person financially or help them get a loan of some sort.
Although opportunistic infidelity often begins as a simple opportunity to get ahead, it often evolves into something else. Into emotional and sexual infidelity. For instance, someone who may decide to sleep with their boss to get a raise may get fully involved in that affair and then leave their relationship for this affair.