Democracy is more than a cross by a name every few years. Democracy is government by the governed - individual self-determination and freedom of movement; workers', consumers' and community control of industry and public services; communal control of land and resources; residents' control of housing; universal access to the means of subsistence, since there is of course no more authoritarian act than to deny a person their right to choose continued life; an egalitarian distribution of wealth, which after all is power.
But even if we were to limit our definition of democracy to the free election of state officials, it would still be a farce to call elections within capitalism democratic. The odds are heavily stacked against the left by the monopoly of the corporate media, which gives the ruling class both the ability to define the acceptable limits of public discourse and an automatic monopoly on election coverage - which makes the vast majority of nominally democratic capitalist parliamentary regimes a one-party state, by no-platforming any candidate who does not conform to the narrow capitalist paradigm of acceptable discourse, leaving those who remain politically similar enough that they may as well be a single party. This disadvantage is often compounded by obstacles such as unproportional electoral systems, bureaucratic hoops to jump through such as large deposits to be paid by all candidates before they can get their names on the ballot paper, and directly fraudulent tactics on the part of the establishment such as ballot stuffing or locking people out of polling stations - and even, sometimes, explicitly banning candidates who advocate revolution.
This is not to say we should boycott capitalist elections. The 'electable' candidates may well all be effectively of one party, but there are differences within all parties; just as there are differences within the organised left, there are differences within the ruling class, all the more accentuated by the inherent competetive nature of market capitalism. It's impossible to know how many years or decades it will be before we can overthrow capitalism and establish true democracy. In the meantime one of the methods we can use to restrain the worst excesses of the ruling class - although we should focus on more direct, militant and participatory methods - is to influence through elections which members of the ruling class make the rules, to keep the hard-liners more sidelined than they might be. There are also rare occasions on which revolutionaries can make a breakthrough into the institutions the ruling class calls representative; if not to gain legislative power (we will likely have the strength to overthrow the state long before we have the strength to take it over through the ballot box, given the measures to which the ruling class resorts to keep us out) then at least we can gain a parliamentary platform from which to promote revolutionary ideas.
It must of course be up to the organised left of the time and place to judge collectively whether to seek this opportunity or to hedge bets and use our electoral voice to keep the state in the hands of the relatively moderate sections of the ruling class. Indeed there may also be circumstances in which it actually is more appropriate to boycott an election.